Hello, I am totally new in this, but I am hoping someone here can give
me some tips.

I have a R/C car reciever (Kyosho Mini-z) which has become very poor
range recently. I have checked battery both on the transmitter and at
the car, crystal, and wiring, they all seems to be fine. The symptom
is if I have both the transmitter and the car switched on, the car will
twitch (the servo) badly, and the motor is start to running on its own
without me pressing anything.

All my electronic knowledge are only from my classes during college,
but I don't have any troubleshooting experience at all. The only test
tool I have is a multimeter, can only test resistance, diode, and AC/DC
voltage. So, I am hoping if anyone can tell me how I can go about to
find out what component has become culprit. And of course the next
question, would be where to buy replacement.

Many many many thanks!