Pontiac Repair: 95 pontiac 3.8l slightly rough idle - bad vibration at highway speeds in top gears, pontiac transport, coil pack

Question
My wife has a 95 pontiac Transport with about 130000 mile on it. The car starts great. But somtimes dies or developes a little rough idle. At highway speeds say 55 to 75 in top gear (D) well either of the "D" positions the cars starts to vibrate sometimes to the point of it being harsh. If I put the car in nuetral at speed it stops shaking, and if i accelerate hard it stops. She recently had the car serviced for this and she said the replaced the plugs and wires and she seems to think the problem went away for awhile. but this service was only done a few months ago. I am thinking perhaps weak spark in a cylinder or plugged injector? Could an EGR valve do this? or am I missing the boat and is it somehow suspension or transmission related?  Thanks in advance for your help.

Answer
You probably have a bad coil pack. The problem was probably temporarily covered up by replacing the plugs and wires.
A plugged injector will not usually go away when you accelerate, it gets worse, but you should have the injectors cleaned anyway. You are supposed to have them cleaned once a year.
The EGR valve should not cause the high speed miss but it can cause the rough idle and intermittent stalling if it is sticking open.