Sounds like an oxygen sensor or MAF sensor issue, but honestly it could be a lot of things. (anything to do with air or fuel delivery or mixture, basically...or, hell you could be losing compression) Is your check engine light on? Have your ECU checked regardless. There are two ways to fix this. If you're not equipped to diagnose it, take it to a mechanic (or at least Autozone to get the code pulled for free...but there's a chance they might not find anything). Or start throwing new parts at it. Hmm...actually, if you take the second route, you should have the parts place pull your codes anyway.