Well, it could be several things, the least of which is the radio system. If you properly range tested it, it should be fine. However, it is not out of the realm of potential problems. Before going to the radio, though, check a few other things, such as:

Other things to look at would be the battery, meaning the condition of the battery. What size is it (number of cells, type (NiCd, NiMh, other) and capacity). Also, was it fully charged? I know.. some are basic questions, but they are all things that should be looked at.

Next, did you check to make sure you don't have binds in any of your linkages? If you have a bind, say in the throttle (go to full throttle and the servo wants to move further than the carb arm will allow it will cause the servo to draw way to much current and this could possiby cause a marginal battery to fail).

What was the orientation of the antenna on your transmitter? Were you using it with the tip pointed at the aircraft or perpendicular to the aircraft travel? The object here is to have the antenna perpendicular to the aircraft at all times (of course, at all possible times.. granted, some times it can't but most likely that woudl be when it is near you and should not matter).

How did you orient the antenna (s) in the airframe? If using a dual antenna setup, they should be oriented differently, meaning one horizontally and the other vertically, and as far apart as possible.

Check this out and get back to us.

CGr.