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Timing belts skip and jump sometimes - you'll be sitting at idle and suddenly it'll die, or just about. This will happen when you set your manual tensioner too loose, or when your belt gets really old and stretches a little. Wouldn't explain why your spark is weak, unless Athal's right and you checked your spark on a battery that was still strong enough to turn the motor and actually fire a spark, but not strong enough to make a hot blue one. A camshaft that's off a tooth or two will still run and can be driven a few miles, but runs very poorly. Mine ran very hot when I asked it to get me home one winter night.
Of course, timing belts break too, but then you wouldn't get a spark at all.
Ground is also an obvious choice, but if you've got enough ground to crank the motor, you should have enough to spark the plugs. Check the ground at the back of the head anyway, but lots would have to go wrong for this to matter, IMO.
I think that when the ignition module itself dies, it dies outright. We had a situation in Dale's car once way back, where it would run fine in vacuum but would buck and fart and detonate under boost. Changing the coil/module assembly cured it, and changing it back brought it back. We never isolated the cause, but we know it was either the coil or the module. In your car, the module should be attached to the windshield washer reservoir bracket, coil is mounted elsewhere.
I once had a situation where my car started running awful, and I traced it down to the wires going from the dizzy to the module running past the alternator, picking up EMI. My alt was in good shape (used it for years afterwards) and simply re-routing the wires eliminated the problem. I'd just worked in that area the night before so I figured it out pretty quickly. I doubt this is your problem, I'm just mentioning it.
Ignition switches start to get crispy with time too, and they do all sorts of funny things: in "start" it'll crank the motor and might run some accessories, but won't run the things that come on in position II, i.e. your ignition coil and fuel pumps (fuel injected models). In position II they will sometimes fail to run anything at all, but work fine in position I. Sometimes you can tell yours is gone by being able to get things (like your dash) to shut off while the car's still running by wiggling the key a little, but this isn't a guarantee. Again, you probably wouldn't have spark at all if this was your problem, but you should be aware of it.
Fuse boxes can and do get really gross down there, right by your feet, down near the salty wet floor mats and all that. You should clean it regularly with your favourite method: deoxidizing spray, bristle brush, what have you. The whole fuse box (and practically all the accessories in the car, including the headlights) is fed by the ignition switch, which is fed through one 10 ga. wire that's spade-connectored to the jumper block behind your battery - clean that up while you're at it. If you've got something that doesn't appear to work, take off the fuse box cover and give the fuses a spin - that usually cleans off a spot so that stuff starts working again, and you'll probably feel one or more warm fuses. I've seen plenty of the factory voltage gauges show 2-3 volt fluctuations when the blinkers are turned on - a sure sign that there's a current flow problem somewhere. When everything's in good shape, the needle doesn't hardly move at all.
While we're on the subject of reliability and electrical maintenance, I like to relay the headlights in my 240s. The headlight power runs from the battery, through that junction block, to the ignition switch, to the fuse box, through the headlight switch, to the high-low "relay" (really a switch), and to the headlights, then usually to a crusty crappy ground, all through spade connectors. Either install a proper relay before the high-low switch (or one each for high and low after the switch, my preferred method), and feed them right off the battery, through a circuit breaker or fuse, and make sure the ground is in good shape. Use good thick wire too. This does a few things: it makes your headlights brighter; it keeps your headlight switch from melting (which it WILL do); and it reduces the load on the fusebox and on that 10 ga. feed wire. I also like to re-route the power to the headlight switch (now only operating the relays, so 1/2 amp current flow instead of 10-15 amps) through the fusebox controlled by the number II position of the ignition switch. You only have to do this in pre-'86 240s, as they're switched in the later ones.
Oh, and has anyone mentioned the biodegradable wiring harness? Yours appeared to be in pretty good shape when I looked at it, so it may have been rebuilt or replaced, but you never know. Typically Volvos before mid-'88 have wiring harnesses that shed their insulation with applications of heat and oil, two things that are abundant in Volvo engine compartments, keeping in mind that the harness runs right up against the block just about everywhere. Luckily your car doesn't much rely on electricity to run...
That's enough for now.
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