Does this sound like driver overheating?

I'm noticing a strange behavior on a gen6.d board. Admittedly, this board was exposed to dust and rainwater, which may have damaged it, but it seems to be working at the moment.

The strange behavior is that the Y-axis motor works most of the time, but if I leave the motor energized without moving for several minutes, then the Y-axis motor is no longer able to move and only makes buzzing sounds. I suspect the driver for the Y-axis motor may be overheating in this condition, and am wondering if this sounds like a plausible explanation.

Here's what I observed:

1. Immediately after power-on, the following G-code properly moves both the X and Y motors.

g28 x y
g0 x100 y100 f3000
g0 x5 y5 f3000
[repeat several times]
m84

Running the above repeatedly never results in any failure of movement of the Y-axis motor.

2. If I eliminate the M84 at the end (leaving the motors energized), then after a few minutes the Y-axis motor can no longer move and only makes buzzing sounds. The X-axis motor still can move normally.

3. In the above condition when Y-axis motion fails, measuring the voltage across the Vref trimpot for the Y-axis motor driver yields abnormally low voltage, maybe 0.3 volts or so (it should be around 1.7 volts).

4. Shutting off the motors (with M84), then adjusting the trimpot through its range cannot recover the voltage to the nominal Vref=1.7 volts (though very rarely it is possible). At one particular setting of the trimpot, the Vref voltage actually appears to be oscillating between 0.2 volts and 1.3 volts (as measured by a digital multimeter with slow update, so values are approximate).

5. After powering off, waiting a few minutes, then powering on again, it is possible to adjust the trimpot to achieve 1.7 volts Vref for the Y-axis driver, and everything works again.

It almost seems like this is some sort of overheating issue, where leaving the Y-axis motor energized without moving for long periods of time causes the driver to enter some sort of abnormal (overheated?) condition where it is no longer able to supply the required current. That would also explain why a power off, wait, then power on cycle allows recovery of Y-axis motion, because perhaps the driver chip then cools down.

I'm hesitant to repeat the failure-mode test (leaving the Y-axis motor energized, without moving) several times, since if it is an overheating issue, I'm sure it's not good for the driver.

Does anyone else have any experience where leaving a motor energized without moving for long periods of time caused the motor to temporarily be unable to move? Could this lead to driver overheating, thermal shutdown, and unusually low Vref that is only recoverable by powering off and waiting a few minutes?

I suspect that you have a bad solder joint somewhere on your board, especially since the board was exposed to corrosive conditions (rainwater). The Vref pot itself may be corroded. As the board warms up, the bad joint goes high resistance, and the Vref connection gets wonky. I doubt that the driver itself is overheating and shutting itself down, from your description.