Mower has 500hrs and has been running hot since middle of last year during cutting season, but babied it until I could get time to troubleshoot. Recently I changed out the transmission fluid & filters, oil and oil filter, both air filters, and drained and installed new radiator fluid. Mowed for about 45 mins and gauge started to go towards red line, stopped and lifted cover and heard fluid boiling. Let everything cool off and two hours later tried mowing again, only got about 30 mins in before the gauge went towards the red again. How would I know if the thermostatic is the cause? Can flush and readd fluid, but did this last year and nothing has changed. Any advise would be appreciated. Thanks.