I had a similar problem with a Humax HDR Fox-T2 and a Sony Bravia TV, where the problem was getting steadily worse over a period of months, up to the point of getting a green screen nearly every time. Like you, this led me to the conclusion that the box was dying. This seemed to be confirmed as a HD Fox-T2 (the single tuner version) worked fine every time with the same TV.
After trying every trick in the book in order to solve the problem,I persuaded Humax that the box was faulty and they agreed to a replacement. However, before this could be arranged. due to a household re-arrangement, the Sony was replaced with a new Panasonic Viera and the Sony/HD Fox-T2 combination relocated elsewhere. The problem went away completely and three years later, it has never happened again on either set up.
I suspect that the root cause of the problem is down to marginal timing in the handshake protocol, making it a matter of luck whether particular combinations work together reliably or not. This may explain the intermittent malfunction and why some people have problems and others not, even if they are using using the same equipment
I realise that this probably doesn't help you and unfortunately I can't comment on the DTR-T2100 500GB as I have no experience with this model.