Pontiac Repair: 1995 Pontiac Bonneville, intake manifold gasket, vacuum leaks

Question
Dear sir: My son has 1995 Pontiac Bonneville (3.8L turbo charged?)
The car runs fine but randomly the RPM's drop and car almost stalls (never does) and then jumps back to normal(chimes sound during this sudden drop)The car does not stall, and no check engine codes are ever stored. This problem is not everyday and just happens from time to time.....some times it does this while driving 10 miles and sometimes it happens after a 100 miles. I have replaced fuel pump, water pump, oxygen senors, plugs, wires. Not sure if this caused by a transmission problem or engine computer problem or some other sensor.
Any ideas or where to start looking would be appreciated.
Thanks you so much!
Mark

Answer
Hi Mark:

that engine is supercharged if it's an SSEI. those engines had issues with vacuum leaks cause by either the supercharger to intake manifold gasket going bad and also ther was a vacuum line under the nose of the supercharger that would split and crack and cause a vacuum leak as well. any kind of a vacuum leak will caus the isle to be erratic that along with different ambinet/engine temps will cause an erratic idle. that is where I would start looking first. good luck :)