It might be many things: media, firmware of your standalone, both, burning software, etc.
I had a similar problem, to my mind it is the firmware of my standalone. The written disk plays flawlessly in my PC, in my XBOX, in my PS2, but the last chapter is a problem in my Philips DVP720SA.
So, in my case I could exclude: media, burner, software, remained the standalone.
Cleaning does not help, though useful.
My suggestion would be to use good media (TY, Verbatim, Ricoh, etc.) and to try another software, like AnyDVD with CloneDVD2 (www.slysoft.com).