Hi, I'm Nathan.Sorry to hear that you're having trouble with this but I'll do my best to help you.
Being on an unlevel surface could cause issues if severely tilted, but that is not the likely cause of the problem.
This type of problem is most often caused by a contaminated laser lens. You can attempt to clean it by blowing out the disc tray/slot area with a can of compressed air for dusting computers (keep the can upright only so you don't get chemical propellant on the lens).
If this doesn't help, you can try a specialty cleaning disc (Allsop 23321 Ultra Pro Carbon-Edge DVD and CD-Drive Cleaner) which has little brushes that clean the lens more thoroughly.
If neither of these cleaning steps help, then this is more likely a hardware fault, which could include a bad laser emitter or pickup, faulty spindle motor or laser transport, bad d/a converter, etc. Any of these issues would require repair/replacement.
Let me know the results, and I'll advise further, if needed.