Last night I parked my '91 745 turbo in the driveway because my wife's new Camry got to use the garage (at least for its first night home). When I started the Volvo in the a.m. it ran very rough at around 1000 rpms, then the idle dropped suddenly to below 500 and down to about 300. Didn't stall, even when I put it in gear. It was running way rich, with black smoke coming out of the tailpipe. I turned it off and started it again three times and the same thing happened each time. But later in the day, after leaving it in the driveway all day, I started it again and it was back to normal, idle smooth, starting out at around 800 and then as the car warmed up coming down to about 650-700. I am very uncomfortable with these intermittent problems because you never know when one is going to surprise you.

Now, just so you know - the car has a new TPS, IAC motor, engine temp sensor, O2 sensor, RPM sensor, and a used ECU (which didn't make any difference to the low idle I was trying to fix). The TPS was carefully and precisely adjusted to the specs in the shop manual, and it is clean as a whistle. The vacuum lines are open and clear. The oil trap is open, and the oil filler cap passes the "jiggle test". So I'll be damned if I can think of what this might be.

What should I do? This is beyond my expertise (such as it is). Thanks.
--
Darius in Albany NY '86 744 GLE (174K); '91 745 Turbo (164K)