I'm sure these have probably been looked at, but just in case.
1- Make sure the fan is correct for the machine. If the fan is of the wrong rotation it can not move air properly.
2- Makes sure the fan shroud is installed, positioned properly and it is correct for the machine.
3- Check for worn drive belts and pulleys. Worn belts and pulleys allow the belts to bottom out and slip under increased load.
4- Use a 50/50 mixture of water and anitfreeze to get the best heat transfer.
5- Verify that your gauge is reading correctly.
Hope this helps and Good Luck.
Tom