[Richard]

Do the following easy diagnostics.

Run the front unit on low cool.
What is the amperage reading on the amp meter at the driver console.
What is the air discharge temp measured with digital thermometer. Cool is not a number I can use to help you.
Let it run for the two hour period you mention.
Did the fans shutdown?
If they did, what is the amperage now?

Now, and this can be a different day.
Run the same test with HIGH cool.
Record the amperage. You should have amperage on BOTH legs now.
What is the air discharge temp?
Does it shut down again after a couple of hours.
If it did what are the amperage readings now?

I know this is slow and laborious, but you are describing a problem that is not evident given the description. It may be a simple fix, and it could be tricky to diagnose. Let’s get the data to help.