M52 2.8 high erratic idle

[Be30]

Hi everyone
Recently purchased an e30 touring that has previously undergone surgery and had an m52 dropped into it from an e36 328i auto. I bought it incomplete and it needed the wiring and cooling system finishing. I have now completed those tasks (I think) and it starts up and now runs.
However, upon cold start it idles around 2,200 rpm (according to the diag when plugged in, not the e30 Rev gauge as that reads higher due to the wrong coding plug)

Over some amount of time this high idles then drops and fluctuates between 1,200 and 1,500 rpm. I’m not too sure what it could be... the ecu isn’t throwing up any faults that are of concern..
So far I checked the TPS, chased and fixed air leaks, lubricated throttle body and made sure it’s closing properly.

Also, if you rev it the revs linger for quite a while before dropping.
Next thing on the list is to check and clean the ICV... just thought I’d post this to see what other people are thinking?
Cheers