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Intermittent Pontiac Montana engine chugging/dying

We have a 1999 Pontiac Montana with around 120,000 miles. It has been a champ of a family vehicle. Over the last 6 months it has developed an intermittent engine roughness that seems electronically-based. First, it is NOT THROWING ANY CODES to trigger the SES light. When the problem seems to switch on the engine chugs like it is not running on all cylinders. Although never totally repeatable, it happens more frequently when in warm conditions, and most consistently when sitting on an incline or decline. When it is occurring, it is impossible to keep the engine from dying on a decline, but let the car roll to a level spot, and it starts up and runs smoothly. Often the stumble comes on acceleration from a stop - but not always. Occasionally we see a TCS and ABS light with the roughness - but not always.

I personally inspected the rear tailight connection boards for a melting crossover problem SB and both looked perfect.

We took it to a Pontiac dealer for a paid diagnostic, during which said the PCM was OK and supposedly checked for the C305 connector problem (an SB for pin corrosion/separation that causes these elect problems in many GM vans). Over several visits and several hundreds of dollars to the Pontiac dealer over a months time, we paid for the following based on their claims of code “history” (?): New Throttle Position Sensor (even though already changed with no improvement), Cleaned MAF sensor, new wheel speed sensor harness, new positive battery cable. With so many visits and no improvement, it became clear the dealer was just fishing and changing parts. The last straw with the dealer was when they then claimed it needed a new PK3 ignition switch core - despite no related THEFT SYSTEM light ($575) and a transmission Pressure Control Sensor ($900 minimum). On that, I took it to a reputable transmission shop that said verified the engine dying was NOT due to the transmission (not a surprise since it would die on a slope in neutral). They were able to reproduce the problem on a slope, ran a thorough engine diagnostics over several days, ruling out fuel pump/flow issues but could not find the source of the problem - so kindly didn’t charge us. They are thinking it is an elusive ground or short, and recommended a local independent engine electrical expert that works out of his home shop. Our car is sitting there now, but he may not be able to get to it for weeks. All have said that most ignition failures should be flagged with a trouble code, but none exist.

This is driving us crazy. The van has been well cared for, and is otherwise in great shape. We have shopped the current minivan offerings and none come close to the utility and features of the Montana. It suits our family perfectly. Between the intermittent problem episodes, it run smoothly, so we hate to scrap such a great vehicle. Any helpful ideas would be welcomed.