If you think it's loading the dice, you should record all the throws (both yours and its), and run a simple statistical analysis of it: are high numbers more likely to be thrown by xgammon than low numbers? or are low numbers more likely to be thrown by you, when playing xgammon? Are doubles more frequent than 1 throw in 6 for the computer?

Or, since this is free software, you could read the source and compile it yourself. Heck, you could give yourself all double-sixes!