hello dave and welcome to wattflyer. i don't know whats up with your setup,but could you share the prop size your spinning? have you done a watt/amp meter to the setup? i also fly the funjet ultra using a headsup rc powerup jet 450 brushless and 4cell 2800mah 30c zippy using the caslte creations ice 100amp esc. this is overkill but its what i had in the drawer and i may switch things up to go 6cell later. one problem i had was a bad zippy pack that must have been sagging in power during wot causing the motor to cut out and need rearming[lvc].most likely a bad cell.
that pack swelled up after a few cycles and has been trashed. i hope you fix the problem and please let us know what you find out. stu