I would be looking at the transmission fluid and filter screen and make sure the inching is adjusted properly. I have had this problem and figured out that the operator kept "resting" his foot on the inching pedal all day and didn't realize it.

Make sure the fan is turned the right way and you didn't say if the water pump had been replaced or not, I'd start there with a new water pump & thermostat, make sure you bleed the system also of you'll have the same problem again real quick.