Are you sure you haven't got a leak in the exhaust system? To check start it up, let it tick over for a minute or so (not too long) and then place the palm of your hand over the end of the can. Obviously if you leave it running for too long you'll burn your hand, don't do that.
Any leak should be obvious as a gas escaping sound from anywhere on the system. If everything goes quiet, no leak is evident and it begins to labour and eventually stall then it's not that, worth dismissing early on though.
A crack or loose connection in the system could cause the symptoms you describe.