I had the same problem only reverse deteriorated over time. With some ESC's setting the end point in the reverse throttle to 80% helped. My other ESC seemed too sensitive and I could only use the GT3b with no glitching by disabling reverse on the ESC.

In my case it turned out to be the transmitter has a faulty reverse. You might want to test channel 2 with a servo and see how it behaves. On mine it only has about 20% travel in reverse and jitters terribly. If it wasn't over 30 days old I'd return it and get it exchanged for another one.