I have been experiencing a confusing problem recently with my otherwise very good used Opel.

For about 2-3 months, the car has run cold on downhills (down lower than 80 degrees) or fast straighaways, at which point the engine will run very rough, and then eventually stall when RPM dropped while slowing for a light or turn. This will not happen in traffic.

Just yesterday, though, on a long drive, I repeatedly turned on and off the AC, and miraculously, the car stopped running cold, and performed jsut as it should for the rest of the drive (even without the AC on for most of the drive).?

Thanks,
David, Jerusalem

Is this a thermostat problem? MAF? Idle setting?