idle speed too high

hi all. in my mk1 i have a 1991 polo GT 3F mpi engine running all factory wiring, pumps, intake management etc…exactly how it was in the polo
had it in there 2 years now and done everything possible to it from skimming the head, port and polish, tubular 4 branch, de-wedge throttle body, raised rev limiter and so on….

recently it as produced an idle speed  problem. randomly..irellivant of throttle position, engine temperature, outside temperature, loads, fuel type day or night. it will either hunt like crazy or mainly sit t roughly 1800rpm. this is annoying and embarresing obviously. nothing i do stops it..shutting it off, stalling it, flooring it…nothing

i have replaced blue temp sensor. checked the entire loom, full electrical check on all components..so far no luck..everything is as it should be, even the dizzy is the correct one. it has been ages since i have done any sort of modifications to the engine, the last being 4 months ago, the 4 branch and gti exhaust so it has nothing to do with what i have done

does anyone have any ideas????

im no expert on these but symptoms sound same as what you get on a digifant mk2 if the idle switch isnt adjusted properly, or the wiring to it has snapped