renault clio stalls and over revs in neutral?

few weeks back my clio 1998 broke down,unusally had put £10 of super in rather than standard unleaded,oil was fine,no gloop,etc broke down after trip 30 miles or so, stalled and lost power usually low geers/speed and over rev at 3000 in neutral.rac came out said exhaust was gone,so replaced middle and back,garage ran it and it passed mot all fine,and when got it back, after 20 mins idle in traffic its playing up again...not all the time...intermittant problem so garage charging me and cant find anything