I hate it when a new plane does this stuff. I suppose you're at the point where the engine will need to be running while you unplug one servo at a time to locate or eliminate each and every one as apotential issue. Then you have to try a different pair of batteries, one at a time, on the Rx and ignition to see if that's it. Of course you've verified that the spark plus is a resistor type. Ignition battery is new and fully charged? 800mAh or larger? As previously noted, good switches? No loose connections at the ignition plugs? Not using any mylar type tape to hold the antenna on the fuselage? That's been a biggie more than once, to the surprise of a few. Rx crystal taped into the RX to prevent vibrating out?

Good luck but I'm sure you'll find it.