I run a GTB 6.5 in mine & gear 19/84 (purple spur). Car has never thermalled, and is crazy fast. Tons of low end grunt also.
You may be geared to low & over-revving the motor. Toss a 20T pinion on there and try it again.
--Joey --Venom Batteries & Chargers!!
Old School Racer & Vintage RC Car nut
RC10CE (X4),B2,B3,B4,B5, B5m, B6D - 10T,T2,T3,T4,T5M - B44.3
BRZ-444, RC10M, RC10L3 oval, L4 oval X 2