Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble, Steve.
This type of problem is 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.
I am an independent technician and I've spent my personal time, unpaid, in an effort to assist with the question for which you solicited my services. This site works on the honor system, and to that end, I have prioritized working with you ahead of other paying customers.
I am only paid for my efforts when customers such as yourself leave positive feedback for my work. Again I apologize there is not a better outcome here, though this is really beyond either of our control. I do hope you’ll consider leaving a positive rating for me, rather than “shoot the messenger”, so that I may get credit for my time spent in good faith to answer your question accurately.
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.