I tend to have problems when I'm using the t-probe and there are fluorescent lights nearby. I'll shut the lights off and the t-probe works better, but I need to see!
I just spoke with one of the Leica techs and he said he has seen this issue before.
It's something that's unexplained (even from Leica). Possibly, it's caused by the particular frequency of the light bulbs you have in your area because i can guarantee you the leica trackers are calibrated in a room with fluorescent lighting.
I've had this issue before with an AT-901 and an older T-probe, all of my other co-workers though I was crazy. What's odd is that it was used in the same room with the same lighting all of the time, but if it would placed in a specific spot in the room, it would have trouble taking points. I have not noticed this issue with the AT-960 and later model T-probe.
I have been to the lab where they are calibrated, and it is lighting by florescent bulbs, nothing special.