overevving pug 406 18i(99 plate march)?

oops i think i put my question as an answer to tims problem sorry tim! my 406 is over revving i cleaned idle valve as instructed and butterfly seemed pretty grimey popped it all back on and hey presto it sat nicely at 850 rpm bearing in mind it was revving to 3 grand before i started so pleased with myself i went for a drive all good still then today it started again ive never changed plugs ht leads etc could this affect it in this way? could it be an ecu prob?