whats the serial number?
I can give you more specific info with that

Sounds like a bad travel potentiometer to me from your description

check the neutral voltage setting and make sure it is at 2.50v in neutral. Also you can monitor the voltage sweep and see if there are any bad spots in the volt range sweep.
you can go into the maintenance menu and run a learn on the handle to verify it is set correctly and calibrate it.

Now unlike the raycat trucks these mitsu's have a different method to get to the learn mode. You will need the troubleshooting manual for this.
Good Luck