I've spent a lot of time reading existing threads but haven't found one that adequately addresses this problem.

Cold start works as expected. After 5 minutes idle settles at 750 rpm. After 30 minutes idle drops to 500 rpm and fluctuates between 400 and 600 rpm. TPS adjustment checks out ok using the single lit LED method.

Any ideas as to what could be causing this behaviour?

 

Vacuum leaks or the TPS are almost always to blame with a bouncing idle. Setting the TPS is one thing, but testing it is another. You should be sure it has the proper resistance values when open/closed, and that there are no dead spots in the sweep. The resistance values are listed in the Series 5 FSM. Links are in the 2nd gen FAQ if you need one.

 

I didn't mention that I had tested the TPS resistance. It wasn't exactly the same as the specs but I thought probably close enough, since testing the TPS setting with the LEDs showed that the setting was ok. The resistance sweep was smooth from closed to open throttle.

I thought the fluctuation at 500 rpm may have been due to the lower lower spec idle, maybe not.

I am puzzled as to what would cause the RPM to drop from 750 rpm after runnuing at idle for 25 minutes?