Backfire

That sounds like an ignition related misfire, how old are the spark plugs and wires? The wires themselves are free to test if you have a spray bottle and about 5 mins after dark all you have to do is warm up the vehicle for a couple mins then mist all the spark plug writes wires with water and look for little sparks/arcs of electricity. if nothing have a helper snap the throttle open and closed quickly, this will simulate load and is more likely to produce results. Even if they pass this test that still doesn't mean they're good but it elimanates that as a problem. You can mist the ignition coil and it's wire also, due to their location they're prone to wearing out. They happen to be cheap and easy to replace also

Is there a check engine light? if so, have the computer scanned for codes. A bad oxygen sensor can cause these problems, but worst case you could also have an exhaust valve sticking. if not an o2 sensor then you will have to take it to a mechanic and have a running compression test done. Lets hope for the O2 sensor.