Just one more thing to check. I had all new kit in a B4 at a Newbury event. It had all the symptoms of the above. Due to the time between races and running two classes, I could only change one thing and test in a heat. Needless to say I only figured it out in time for the final.
Turned out it was the Banana (Corally) connector that went into the LiPo. The spring gripped the inside of the LiPo fine, but it wasn't gripping the pole and therefore intermittent connection.
Worth checking that first.
Also, I have used many different Spektrum receivers. The 3500 Micro was fantastic, as I could tuck that in with the rest of the wires and didn't have to worry about signal loss or having an aerial. With the new DSM2's I had loss of signal and had to install an aerial tube. Also, I noticed on the instructions to not put a cap on the tube and have the bare cable section protruding.
The above costs nothing and whilst it's the most unlikely, it's worth checking, just in case.