strange glitching problem

[nivlacf1]

so now the cars and drivers are getting better our racing is really staring to become fun, some great help from people at our club and the kids are doing really well,

BUT we now have a really strange problem on one of the cars and no one seems to have any answers,

every 30 seconds or so it will suddenly glitch and go backwards not a lot may be half a wheels turn or something, i have tried changing all sort on the transmitter and esc but to no avail,

Anyone ever had a similar problem and found what it is/was? via fault testing i am thinking its either the esc or transmitter but both are a lot of money to spend if they do not resolve the problem,

i look forward to any input