I wasn't sure where else to turn with this, but I figured since the GP has used this engine in the past someone here might have an answer.

My parents have a 1995 Buick Regal with the 3100 V6. Mileage is currently about 110K. For the past 6 months or so the car has been hesitating and shaking during acceleration. After pulling the plugs last fall, I found that several of them were showing a large amount of ash deposits. I replaced all of them with OEM spec plugs, replaced the wires and dumped in some fuel injector cleaner. This seemed to help most of the problem but didn't cure it completely.

The problem started getting worse recently, so they took the car to a shop and the mechanic once again changed the plugs due to major ash deposits on the plugs. I have had them change gas brands, addtional bottles of fuel system cleaner, etc., but with no improvement.

Any ideas?

Changing what brand gas you use will make no difference. It sounds like it is running rich.

It doesn't seem to be from what I have been able to tell. There is no noticable smoke of any kind from the exhaust, and fuel economy has been normal. I had suggested they change thier brand of gas when I learned they were filling up at a local Wawa convenience store, most of which tend to use unbranded gas of (occasionally) questionable quality. Also, I checked some service manuals I had for other cars and compared the plug to the photo chart in the rear of the book showing the different ways plugs can foul. The comment for plugs with ash deposits suggested that the condition could be caused by additives in the gas or oil, so I figured it was worth trying.