This weekend I was up at Watkins Glen and my G2X was basically unreliable and unusable. Its been rock solid for the last several years and I hope somebody can shed some light on where to look.
A typical lap time for me is in the area from 2:20 - 2:25 minutes. I set S/F in the first session, and it seemed to be working ok. It started a lap time when I came around for S/F the first time, the rev lights worked correctly, proper gear displayed, etc. The first session I wasn't paying that much attention to the times as the track was wet and I was focusing on relearning the line, etc. After several laps I looked down and it was telling my my lap time was in the area of 7 mins. After the 20-25 min session it had recorded only 3 laps, one only 4 seconds long. Predictive timing didn't work. Back in the paddock it was showing from 8-9 satellites. 2nd session just like the same, sometimes s/f would trigger the laptime, others not, Once again, when I looked down there was a 3 second last lap time.
I've had similar issues, mostly at the Glen. One of the things Tim and I came up with is that I tended to have more problems with laps running together and predictive not working at tracks with garages.
Since then I've made sure to not start out until the G2X has satellites synced it seems to be better. I've also found that the grandstands at either side of start/finish makes it a little less reliable and that moving my start/finish to before the bus stop seems to help as well.
I've also seen that running with a full card (no empty space) will not allow predictive to work correctly, which seems odd. It may just be coincidence.