Question: Did you unplug the heater or the sensor part? You must unplug the sensor part, which is a single wire. (please disregard if you've done this)
Another idea is to check the temp switch on the radiator. On the turbo cars this switch closes at higher temps and turns on the electric fan and advances the timing (timing advance occurs only at idle, as detected by the throttle body switch). The temp switch in my '87 malfunctioned and would close at too low of a temperature. The IAC would not react fast enough to compensate and I got an erratic idle speed. I didn't like the idea of advanced timing @ higher temps anyway, so I disconnected mine at the EXK117 computer. Also, Check the throttle switch.

Just some ideas, hope they help.

bagle '87 745t just finished some bodywork