Can any of you help me with what to do next? I'm at a loss to diagnose the problem, or fix it. Details below, and please excuse length of post, but would like to give you all the facts if asking for your help.

Our 1991 745T began to run "rough" last Wednesday morning. To me it seems very clearly like it is either not firing on one cylinder, or something is out of balance. At idle you can clearly feel the vibration, and at idle with the tranny in D, the car shakes to the beat. Whatever it is has a distinct beat and is not the random roughness that tune-up/driveability issues usually exhibit. There are no expensive sounding noises coming from the engine, thank God, just a very regular shaking kind of roughness.

Exhaust at idle does not smell gassy. And it seems to run pretty well at cruising speed. It does sound and feel like it is missing on one (or more) when accellerating from a stop, lacks of power too, at low rpm. When at 40 or 50 or so it seems to have almost all of its former power if floored. As engine revs up above idle it seems to smooth out, or maybe it just seems it as vibes get higher in frequency.

I replaced the distributor cap and rotor, and the plug wires are only a year (about 20,000 miles) old, genuine Bougicord class F.

What I didn't find
Hoping it would be something cheap and easy, I've cleaned the throttle body and idle air control. No improvement. Checked the plugs and wires, they were fine. Verified that it is getting spark at the cylinders. It appears that there are no vacuum leaks. Breather, hoses, and fittings have only small amount of build up and plenty of open passage for air to flow nicely. No water in oil, and no oil in water. No pressure in the radiator hoses until well after it gets heated up, so no pressure build up there from gas leakage across a blown head gasket. No bubbling in the coolant.

What I did find
There seems to be no change in the idle if I pull the #2 or #3 plug wires. But if I pull either the #1 or #4 wires, it will die. The same goes for the injector connectors - it will run pretty much the same with either #2 or #3 disconnected, but will die immediately if I disconnect #1 or #4 (with all others connected). Spark plug for #1 was very lightly dusted in whiteish, but pretty good looking. #2 and #3 were coated with a thin and even layer of matte black, and #4 was kind of a blend of #1 and the other two. None were really awful or anything, although the matte black bothers me.

I did not swap injectors around to see if it is one of them, because I don't have any seals or o-rings on hand in case I screw one up pulling the injectors out. They are all making clicking noises, though. And all sound very much alike, that is to say there isn't one that is barely clicking or something.

I also tried the old "rub a cheap plastic straw or pen on the exhaust manifold" trick and found that #1 and #4 exh. runners were hotter than #2 and #3, and possibly #2 is the coolest of all.

I'm thinking that it is a stuck valve or, if it has hydraulic lifters, a collapsed lifter. The funky beat seems exactly like what happened when a valve stuck in an airplane engine once while I was on my way back to the airport. The only other things I can think of, with my limited knowledge of Volvos, are bad things like blown head gasket (but no oil in water or vice-a-versa), or some kind of piston ring problem.

What should I do next? Could it be a leaking/dead injector? If it is a stuck valve, what do I do? And how do I find out if it is a stuck valve?

Thanks
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)