Well, it could be some problem with the receiver, too. A HW V3.1 shouldn't have too many problems with that servo. I ran a 1257TG and 1258TG with the puny Reedy XP SC700-BL RTR ESC - a few possible brown outs at a grippier indoor track as I recall when used in an SC10, but at a lower-traction outdoor track none - and with the B5M buggy no problems even at the indoor track. The 3A BEC in the V3.1 should handle the 1256TG with relative ease by comparison.

By the way, the RX-37 receiver that comes with the MX-V can actually handle 7.4V, but of course there's no actual need to test at 7.4V. And the V3.1 doesn't do 7.4V BEC output anyway.

At any rate, different servo, different receiver, glitch buster - all good things to test out if possible, as well as the sensor wire EbbTide also mentioned.