My car is exhibiting a peculiar mode of operation... maybe someone can shed some light.

The car is a 1982 turbo; it is water-cooled and intercooled. M46 transmission. 293,000 miles. Other than the later style turbo, everything is factory stock.

It appears that I've got a problem with the idle circuit. It's been doing this (showing this anomaly) for a couple weeks. The symptoms:

1. It starts easily and runs fine while under load (accellerating or just driving down the road). However, while at idle (car is stopped, like at a light), the engine speed either "oscillates" between about 600 rpm and 1500/1800 rpm... or it just idles "smoothly" at 2000 rpm. I've noticed that when it's at the high idle, if I shift into 1st gear, and let the clutch out just a little (loading the engine), the idle speed will be reduced to about where it should be (800/900 rpm). However, this last only a short period of time; it (usually) then goes back into the "oscillation" mode.

2. While driving, when I shift from one gear to the next, the engine speed (generally) does NOT decrease... in fact, sometimes, it actually INCREASES. For example, if I make the 1st-gear-to-2nd-gear shift at about 3000 rpm, the engine may race to about 3200/3300 rpm until I let the clutch out. Naturally, at that point, the engine is forced to a lower rpm; I can then continue accellerating. Same thing happens at the 2nd-to-3rd shift and 3rd-to-4th shift.

3. This one just revealed its ugly head a couple days ago... the engine backfires through the exhaust. This happens when I'm decellerating, in a particular gear. The scenario is: I'm coming to an intersection where I need to turn. I let the car slow itself a bit (it's still in gear; no brakes yet). When I apply the brakes, at the point just a little before the engine would die, it backfires. Quietly; maybe two, three, four times... but it's definitely a backfire. I engage the clutch, downshift, accelerate, up-shift, and... then it does the "engine race" thing I mentioned above.

So... any thoughts?

As always, thanks.