Buick problems

My "family" car is a Buick Century.  It has the 3.1 ltr V6 in it.  The 3.1 is a direct descendent of the 2.8 engine in our V6 Fieros.

Last year, the engine started running rough and misfiring.  No engine codes came up so the dealer did a diagnostic and "found nothing wrong" and charged me for it.  Then in February of this year, the misfiring got bad enough that the car would stall.  Still no engine codes.  Finally in March an engine code came up.  I brought it to a trusted mechanic who informed me that the code only states that the engine was misfiring.  He changed the plugs and the wiring and it seemed to fix the problem, though I sometime thought it was still happening.  Well, last week, the misfiring happened again and no engine codes.  I brought it in and they discovered the the ignition coil was fried so replaced the coil pack.  The car went pretty well trouble free for 2 days and it's misfiring again...still no engine codes. 

Any suggestions?....let's be constructive.  It's already been suggest I get rid of the car.    I can't get rid of the car until the issues are fixed.  It's time to trade it in for something else.  So much for the car that was the #1 JD Powers survey for quality in 1999 and most reliable used car in 2004.

Thanks.

Dr.Fiero wrote: Look for something non ECM controlled. A sticky EGR valve comes to mind.  You could have a chunk of carbon kicking it open every now and then - this causes the cylinder nearest to it to go lean (and mis-fire).

Thanks...this is most useful insight as it seemed to me it was the #1 cyl that had a bad spark plug last in March.