When the temperature outside is in the 80's or over the sliding power side door does not open or close completely. It will start to open but very slowly and then stops half way. What causes this and how do I fix it?

 

The classic issue with the sliding doors is the flex cable. I have rewired one and the thick, black ground wire was indeed almost broken. If you can get a cable from the salvage yard, it may fix your problem (unless this too is faulty). I think you can interchange the cables from left to right to see if the problem moves with the cable. Inspecting it is of course the first step.
As yours moves slowly, it may be that some rollers are stuck. Does the door move freely if handled manually? Some grease to the upper and mid-way rollers can make wonders.

 

The door moves fine if you manually open and close it. It also opens and closes normally when the ambient temperature is under 85 ° using the power button.
When it is hot outside it starts to open but then stops about after it moves 10" and must be manually opened the rest of the way. When you then close it it will move and close completely if you place you hand on the door and slightly put pressure on it.

 

If it's nor mechanical, it has to be electrical. The easiest would be to measure the +12 V feed by the door post between the front door and the sliding door. You can snap off the lower plastic cover and you will find the connector at the bottom, just under the driver/pass belt reel. The 12 V feed goes via the thick Wite/Red and the Black leads. It should be constantly on, at least mine is (after opening a door at least). While measuring the voltage, open or close the door and observe the voltage. It will drop, but only slightly (0.3 V in my case, less if the engine is running and charging the battery). I just measured it going from 12.3 to 12.0 V. If the voltage drops more at this point (say more than 1 V), you can measure from a good ground connection to each of the two to find out which is the culprit.

Next, measure the same voltage at the other end of the cable harness. It can be found on the inside of the sliding door, at the very bottom. As it will move, measuring can be tricky You should see about the same voltages as at the other connector.