Pretty sure that most esc's are least efficient at lower throttle inputs which could be your heat issue..
secondly, are you running a 75mhz? i know that mine screws with my temp gun so if you were holding both in the same hand it could have been a mis-read.
see below..
Benjamin White
R/c Monster Team Driver
Jq the car, LST, Sportweks turmoil pro
Unconventional Techniques, Superior Results