You can try a CD/DVD LENS CLEANER and if no improvement, try another drive. The laser does degrade over time as well as other parts. Repair is possible but in most countries will exceed a new drive's cost.
I have a DVD/CD Reader and CDRW Combo Unit from 1999 and over the last few years it has started having problems playing DVD's. CD's and Writing CD's Works absolutley fine but In recent years sometimes DVD's will play but other times it will try to continously read the dvd, Can it be clreaned or is it just too tired now? since it is 5 years old.