[lincpimp]

Go with 3.1v per cell. You must be hitting the lvc. I have run those cells with good success myself. I have a feeling that you are pushing them pretty hard in a truggy. I run 5s packs of them in my hyper buggy. Go with a 5s pack of larger mah cells and a 2000kv motor, or the cc 2200 motor, great 5s motor, especially geared for 40.

You are geared pretty high, and have lower motor rpms.