Rover 25 loosing power and odd sounding exhaust

well so far i have changed the spark plugs and put fresh oil in and it still looses power but at really random times it has done it first thing in the morning then been fine all day and other times it has been fine all day and i pull up somewhere and when i set off again it looses power no response from the accelerator at all so i have to pull over and turn the engine off and on a few times and then it seems fine... also today it hasn't done it at all but driving home from school the exhaust started sounding like I'm driving a sports car like a deep muffled sound any ideas on what could be causing this?

I have yet to change the coils and fule filter only when funding allows