Intermittent help

[rondoolaa]

We have 5 minute race's in our 14th scale micro buggy heats and every race about 4 to 4 and a half minutes in my car will go slower for a few seconds then speed up again then randomly go slower again then speed up again and so on, it's a pain when your winning a race and then this happens on the last minute and costs you the race. I have tried multiple motors and its the same, the esc is a hobbywing 60A , with overheat switched off, servo is savox 1257mg,

If anyone had any idea what could be causing this please help, thankyou.