Okay so I have a 99 th sport that runs like a dream, at least it did until this morning when the idle fluctuated from 800 (where it normally sits) to roughly 200 multiple times and then eventually stalled. Up until this occurred this morning the car was mechanically sound, disconnected the battery last night to clean that corrosion stuff off and then reconnected and just had suspension, control arms and ball joints replaced on suspension (just in case it's relevant)

Any ideas? I'm stumped

Search the forum for the relearn process the ECU will need to relearn the engine and gearbox after disconnecting, failing that the idle speed controller might be gummed up - search for cleaning the ISC