No Power

[ambulance999]

Please help. I have a 1997 323i 2.5l version. It has just had an m3 exhaust fitted which sounds sweet however I have also inherited a severe problem. When the engine gets hot every now and again the engine revs will drop below 1000 and no matter how hard I press the accelarator the revs will not go past 1000rpm and the top speed as you can imagine is about 3 mph. However if I turn the engine off and restart it the problem has gone and everything is fine untill it happenes again just a while later. There is no consistancy as to when the problem happens. I could be doing 70mph down a motorway or plodding along in town. It just happens at any point. The needls just drops and unless I pull over and restart the engine Im going nowhere slowly!!!!!! Back to the m3 exhaust. I have also noticed a blowing on the join where it is attached to the origional manifold. Could this be the computer cutting in as some sort of safety function or is this just coincidence ?????