Had the same thing happen to me. Mini antennas are great, but placement issues become super critical. I learned the hard way.

Is the antenna mounted vertically on top of the fuse? Trial and error has shown this is the baest placement. If it is parallel to the tail servo wires, then glitching will result. Make sure antenna lead coming out of rx does not go near battery wires or run parallel or in the proximity of the servo leads. Vertical placement makes it roughly parallel to the transmitter antenna most of the time.

Sounds like a very exciting plane. Always great to hear about a conversion that outdoes it's glow counterparts.