It could be caused by resistance from the on-off switch causing a voltage drop.
Just for a quick test, try plugging your battery directly into the receiver and see if your problem goes away.
If the switch is the problem, Futaba makes a nice heavy duty switch that should help.
It is a much better switch for larger planes.
Tower has it but it's hard to find on their web site.
http://www2.towerhobbies.com/cgi-bin...?&I=LXXA49&P=7