I had another go at GameTap. I have found that other games run well.

Still I have decided not to continue with downloadable games. Reasons being that the support is bad, if the downloaded game does not work it is assumed it is your fault and you do not own the game even if you payed for it(you are just renting it). When I buy a game I want a copy, if the disk fails then I can get a replacement and usually the support is (slightly) better. Possibly downloadable games are the future, personally I will wait and see.