Idle speed slow to settle to 850 RPM

[Raggie Rick]

Hi
I Hope somebody can point me in the right drection.

When I have been on the motorways on long a trip and have been pushing the speed this is when I seem to get this problem. When I come to a stop the idle speed stays around 2000 RPM but after a short time around 4 to 7 seconds the idle speed reduces down to close to 850 rpm. On town and country lane this does not seem to happen but it is a little slow coming to the 850 mark (It seems to hesitates a little).
I am running a Mazda MX5 mk2 1.8L (UK) 0n a 1998 plate, M45 supercharge, intercooler, RX8 yellow injectors, dual throttle body, Megasquirt MS3 by Reverent and pushing 179 BHP at the wheel.

Please help.

Rick.