My first suspects would be A.) the "valueline" TYs. I use TY as well, but ONLY the premium disks - not THAT much more if you catch a sale at supermedia or cddvdrmedia or one of the other heavy web hitters. Sometimes you have to purchase 200, 400 or 600 to get sweet deals on the premiums.
I also ONLY use white, hub printable, inkjet surface. I've had no experience, in a long, long time with thermal surface blanks, but there could be an issue with the thermals and possibly lesser quality in the valueline product.
B.) a four-year-old burner could be experiencing a weakening laser head, and that combined with the valueline disks.
If I were to begin experiencing these issues I would first set aside my current stock of blanks and purchase some high quality disks, and try them. Possibly the issue would be solved. My first suspect would be low quality or defective batch of blanks.
If after trying top grade TY blanks I still experienced this, I'd run a laser/lens cleaner, and try again.
If the problem persists, I would think I could safely suspect the burner and replace or upgrade or whatever.
It COULD be a software issue, but unless you are getting some kind of indication or warning from the system, I would doubt that is where the problem lies.
Possibly a re-install of the software????
I was not clear if your clients were the ones claiming the playback problems, or if you'd actually tested the end products yourself and experienced the playback issues. If the complaints come from the clients, I'd find out how old their playback system is, when the last time they cleaned it, etc. People who play a lot of rentals don't realize the potential for oils, dust and other crap that accumulates over time. Dirty lenses there could be the culprit as well.