I had a similar problem with a couple of the DVDs from the TAM 7 set. They wouldn't play properly (I think they played up to a point, then would stop or freeze) on my stand-alone DVD player.
I think the JREF store would probably have exchanged them for another set if I'd sent them back, but since they played OK on my PC I decided to copy each to a blank DVDR, which then played fine on my player.
Bit of a hassle, though as mentioned I would imagine the JREF would exchange faulty DVDs for new ones if you ask. I think they do need to check the equipment they are using to make their DVDs as something clearly isn't working 100%.