Sorry to hear you hvae problems with it, IDK if I'll be much help but here goes:
Lets start from the top-- do you have the correct spark plugs installed? Make sure they are a recommended plug. You don't want to put a plug in there that is too hot, or too cold, you will not get proper ignition.
If you've already checked that- try these:
1. Did you check your antifreeze for oil or any discoloration? If there is oil in it, you could have a head gasket issue. If there is water in you oil you could also have a head gasket issue.
2. Did you check your oil level with the engine cold? if you check your oil while it is hot, or right after running it, you will get an inaccurate reading.
3. Have you checked your spark-plug wires to ensure they are snapped all the way on, so you aren't missing a cylinder?
4. Did you try tightening down the valve cover bolts, and every single other bolt you can reach?
I know they sound stupid, but maybe if i suggest the obvious ones first, you can fix it without getting involved in a major operation.