Usually when this occurs I would start looking at the spark plugs. Reason being is if your spark plugs are fouled is when this starts to occur. The other things you could look at is a replacement air filter, fuel filter and a oil change. If it has high milage I would suggest the Castrol High Milage for the oil if you can get it. You could also add Bardahl high milage when you do the oil change which would also help with maintaining the engine and cleaning up any gunk and carbon build up.
Good luck