Hey John,i know of basicaly only 2 things that could cause the problem you described. 1 is that the plug u are using isent a resistor plug such as the Champion RCJ8. People will tell you that all plugs are resistor plugs now a days,but they aren't! The second thing that could cause ur problem is a bad or weekly insulated ignition kill switch. The kill switch woent show any signs of trouble on the ground,but once the plane is airborne and forward progress is achieved. The ignition system forms magnetic loops that the plane is passing through,these loops will affect the switch and cause radio interference. I had the same trouble with a sach's 4.2 about 3 years ago. Pulled my hair changining everything till i changed my kill switch,then no more troubles.

Hope this info helps you out,good luck.

Art