is this a new install or an old install?
your running 2 motors with one switch, i assume that the switch grounds the motor and when you switch it off the internal park switch grounds it until the motor parks. if the motors are getting out of sync after running for a minute or 2, when you turn the switch off, each motor is grounding the other motor thru the park switch.
what i mean by getting out of sync is one motor is running a little slower than the other one. they both start out going left, right, left; but after while you might see them going opposite of each other or slightly out of whack with each other.
try running each motor separately to see if both motors park right after 2 minutes. if they both work fine then either use: a DPST (double pole, single throw) switch to separate the grounds. or if the switch matches your other interior switches, use 2 relays to disconnect the grounds.
a 1N4001/L diode would work if you had feedback thru a relay. a relay take a small load to energize. a 1N4001/L diode will not take a lot of power, such as a motor load. you could buy a big diode but it would be cheaper to try either suggestion above.
either way write back if this works...