It could be an awful lot of things, from a dirty air filter to a choked exhaust to the bootful of junk you carry around to a sticking brake and everything in between.
When was it last serviced (properly serviced, not just an oil change)?
Is the engine management light on when you are driving? Changing random sensors is unlikely to do anything except empty your wallet.
Start by making sure it has had recent air and fuel filters renewed, and drain the water trap.
hth, November 2009
