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Volvo 850 misfiring

?97 Volvo 850, non-turbo has developed a steadily worsening random misfire problem. A check engine light is eventually triggered and all cylinders are susceptible. What parts are likely responsible for such behavior? Is there a good process of elimination for likely culprits?

The sparkplugs and plug wires would be good first suspects.

Yeah, I agree, plugs and wires. It is possible something in the distributor assembly, but solid state ignition should not have that kind of problem.

I’m not sure if your car has one central ignition coil or multiple, but if there are multiple, failing coils could cause this.

      Thanks Cougar and McLaren Fan!  I have replaced the plugs and wires (actually that was done during the 200K checkup about six months before the problem began showing up).  Am trying to avoid changing the coil if that is not necessary (low budget fixes first, but may through in a used one if I have too).  Also fuel filter and checked out the fuel pump, any other ideas?

What codes are triggered? You changed spark plugs 6 plus months ago. That is long enough for spark plugs to foul. Have you removed them and determined their conditions? Have you changed the fuel filter recently (since the misfire problems began)?
How did you check the fuel filter? Simply sticking a fuel pressure test gauge on it isn’t adequate.
Describe when (idle, cruise, acceleration, decel., engine hot or cold) the misfires occur.

The EGR valve may be dirty. Have you checked for vacuum leaks? I doubt the coil is the culprit but it may be the cause. I would suspect other things first before it.