For four days we ran races with the same laptop connected to a TDC8000+. 520 used as back-up. Tag 553 used on start and and interval 1 (had problems after two days with Interval 1 so we took the back-up start out of the 553 and replaced it with the interval to solve the problem). No issues for 4 days.

Took two days off, and was using the TDC for a Mogul event so we used two 520s for this other race. Everything else was identical. Every racer we were getting false finishes. This didn't show up until we started the race. So we unplugged the lap top and 520 from the wall power (which was powered through a constant power UPS being charged by a diesel generator). Still no fix. Plugged the start and finish in through the 553 as the start was already plugged in through the 553). No fix. We unplugged the 520 from the laptop. Problem solved. Anyone else have a problem like this?
I am going to play with it some more, but really couldn't in the middle of the race.
Jordan

Jordan,
I've ran into the same problem a few years back with the PTB. Tag's reply was we were doing something wrong. Switched to an S4 and never had a problem. Still elusive, let me know what you find, but it does have something to do with grounding. We started using opto boxes for both impulses (all) and opto for computer anytime we ran into a PTB and never recreated the problem.

Agree with Jim that proper grounding might lead you to the culprit. Did you happen to run a ground wire from the Ground terminal on the Generator to a grounding rod? Probably not as nobody every does. (good luck trying to pound in a ground rod this time of year) Maybe try running a wire from the generator ground and connecting it to a snowmaking hydrant with a big hose clamp. (Check with your electrician first!)
How about unplugging the laptop from the UPS and running it on battery leaving it connected to the 520? If this works - no connection to the generator - then you found your problem. How about trying to recreate the problem while having the laptop plugged into a regular 110v wall socket, not the generator?