Quark Woes --- Need help please

[Dafni]

Hey gang. The Quarks seem to give me a headache lately.
I built this other tuck for someone, with the good old 5S/Quark 125B/ 9XL setup.
The truck is rather heavy, 11 pounds I think, but the 5S should take care of this.

Now here's the problem: the truck keeps on cutting out after like 10 minutes of running. Believe me, I tried everything I could: different batteries, 2 different Quarks (same result, even though the second one I tried runs fine in my own 5S truck), all the gearings I could squeeze in there... always the same. Cuts out after a couple minutes. The temperatures seem to be fine.... there's the RCM heatsink and 2 fans on the Quark.

Now I have it down to 110 amp peaks (Eagletree), geared for 35mph, and it still cuts out. That sucks.

What could this be??

I need a solution quick, the truck is for a friend.

Thank you
DAF