With my 89 3FE I have also been dealing with this problem for a long time now and the amount of money that I have spent is ridiculous. In Park it idles roughly at 650RPM, stumbles for its first block driven, once warmed up and I stop at a light it dips down constantly but doesnt stall. It has 171K (perfect compression still almost as good as new) on it and the motor has never been opened. I have given it a full tune-up with all Toyota parts, replaced the MAF, fuel pressure regulator, cold start injector, checked EGR, have found no vacuum leaks, repalced the coil, fuel injector cleaners, replaced the very expensive 3 out of the 4 (one is only for the temp. gauge) sensors around the thermostat housing. One was the temp sensor for the computer, the Thermotime switch, and the third which I have forgotten what it is for. The IAC valve clicks three times when shut off so I assume that is ok. I guess that the o2 sensors are next but since everything is original and really rusted on the front part of the exhaust, the downpipes, Y Pipe, both catalytic converters must be done as well. So for now I will live with this annoying and embarrassing problem. Good luck and I look forward to hearing how it turns out with yours.