nosram comet hd problems..help!!

[quincey]

ive had a nosram comet hd for over 2 years now and its only ever worked perfect once and that was when i used it for the first time,everytime since it has done nothing but stutter when the car is going down straights,ive tried different motors (new and old) new sensor leads ive tried the settings all at basic stuff but it still does it? its been used with reedy sonic mach motors with nothing hotter than a 7.5t in a buggy...its never been warm or used in the wet,i did open it up to find a bit of sand in it but thats it....anyone got any ideas? if not ill have to send it back to schumacher...its only done 1 full meeting and every meeting since then ive had to take it out and run a crappy £30 hobywing combo...