Funny - as part of my troubleshooting I actually re-installed the original blue-plastic cased EI that came with the engine. And the engine runs great! As for the suspected radio interference, the elevators still had that intermittant movement - so either it wasn't the Rcexl EI, or both EI's were having problems. Then I swapped out both of my elevator servos - Hitec 5645MG's - with two Futaba 3305's. And the intermittant movement is completely gone. So looks like the Hitec's were worn (even though I could feel no gear slippage and they worked fine without the engine on). Or could it be that, because the Hitec are digital they were more suseptible to radio or electrical interference?