Once you take amp readings on everything, and if they come up normal there a couple things to consider.
1. Does your client have an irrigation system? Does it only trip after they water or after a rain storm- indicates open insulator jacket- Has anyone been doing any digging since you noticed it started tripping?
2. If not, then the next thing I would consider is the fact that you may have a weak breaker in the transformer. I have had these from time to time (not very often, but it does happen), I was able to replace the breaker in the transformer and no problems after that. These were MDL transformers, so it was easy getting into the guts to replace it. Some transformers are not that way though. If all things check out normal on the amp side, I would be suspicious of the tranny and drop another one on there. (especially from an hour and a half away.)