Pontiac Repair: 94 Olds vibrates when accelerating, auto parts stores, fuel sending unit

Question
Hi Todd,
I have a 1994 Olds Silhouette Van with a 3.8L engine.  It was running great and all of a sudden it vibrates when accelerating at around 1500 RPM's. (it sounds like when you go off the side of the road and ride along the grooved warning track).  It has good torque when you punch the accelerator and also at high speeds.  Last year we replaced the fuel pump and fuel sending unit.  I just replaced the Fuel filter, and cleaned the EGR Valve and EVAP.  Last year we cleaned out the fuel injectors.  I also put a can of Seafoam in the gas and it still accelerates rough.  Do you have any thoughts on what the problem might be?
Thank you for your time!

Answer
Hi Rick:

does this happen more when it's damp out/raining or first thing in the AM? if so I'm thinking of an ignition system issue like a bad set of plug wires or a cracked spark plug. maybe even a bad igniton coil. being the age of this van it going to be harder to find the root cause from 196 and up the computer system are more advanced and record sanp shots when an issue occures these older ones do not do that. you may want to have the van scanned for codes at one of the big box auto parts stores and see if any codes are stored in the engine computer. If there is something stored it would give a place to start looking. Looks like you covered the verya basic things to I think it may be a bit more complex. good luck :-}