Could be a couple things:
1) how hot is the motor and ESC? It could be thermal-ing. Most ESC's have a cut off if the temp is too hot, and will come back on when the system cools down. If this is the case you need to check your drive train for where you are binding up. A silver can at that gear ratio shouldn't be getting hot.
2) weak battery. If it does this within minutes of a full charge then you may not have a good pack. If you have a NiCd or Metal H pack with a bad cell then pack can drop voltage under a load like you discribe.
3) bad ESC. These aren't the best ESC's, I've had problems with the kit ESC, but check the other items before buying a better one.
Welcome to on-road, hope you enjoy it.
"We face each other as God intended. Sportsman-like. No tricks, skill against skill alone." -Fezzik