Buick - Parkavenue :: 1995 Skips Going Uphill Or Under Load

I have a 1995 Park Avenue 3.8 with 278,000 miles. Runs fine until about 3rd gear and 55mph when if under any load at all the whole car jumps and shutters. If not under load, like going up-hill, no shutter or skip and runs smooth. If accelerator pressed at all, even when running smooth the shutter begins. Normally does not shutter at lower speeds.

Thought it was the transmission, so serviced the transmission. No improvement.

Took car to local transmission repair. They drove and do not think it is the transmission. They did check pressure at the throttle body, gas flow - let gas flow into clear container, no debris or water - and pressure was in the 52 range. Dropped some on acceleration while in park.

Shop did scan the computer, no codes. Check engine light is not on. Checked fuel filter and it was good.

They also checked all the spark plug wires and indications were that all were firing and in good order. They misted the wires with a spray bottle to see if they could detect electrical leaks - did not find any.

The car is not driven that often and was very low on gas. After leaving the transmission shop, bought gas and initially the car ran great with no hesitation or shutter even at 55mph. Shutter returned within about a mile. The car acts as if it has time to fill a container with gas it runs fine until the container looses some of the gas then it runs rough, or if you are driving along and try to take more than the normal flow out of the container the car runs rough - if they makes any sense.

Bought new gas filler cap just in case. No change.

The fuel filter and fuel pump were replaced at the same time about 2 years ago.

The EGR was replaced about 1 1/2 years ago. New plugs and wires about 1 1/2 years ago.

There is sometimes, but not all the time, a loud pitched whistle which seems to be coming from the area of the gas tank. Oddly enough if the car is in park and running while the whistle is happening, if the brake is pressed, the whistle changes in pitch to a louder and fuller whistle. When the brake is released, the whistle pitch drops in volume and more of a shrill whistle. Same happens moving transmission from P to R.

I feel I have a fuel problem, however, fuel flow, pressure, etc., seem to be ok. The mechanic drove the car with pressure gauge attached to the throttle body and didn't seem to feel that the gas was the problem upon returning from the drive.