With 10 cycles per burst I set the gap to 1.8ms but I still get the problem. I also tried a 2.63ms gap and it didn't happen as easily but it still happens. I could deal with a gap of 1.8ms but it seems I need a gap of about 5ms or more before it's reliable enough.

I tested with bursts of 100 and 1,000 cycles with a pause of 400uS. This worked for a second then stopped on it's own as with the 600/600 test.