welcome. We need more info. on your mower. Model....engine type and numbers.
Running 15 min. is a long time, so the first thing to check is heat from the engine.
It should have a shroud that may have restricted air flow around the motor and it is
getting to hot. Rodents like to make nests on motors over the winter. That is what
happened to a Opposed twin engine that I had and it snapped a hod!
Get us more info.
Thanks for the quick response. The tractor model is DGS 6500. Engine is 26hp OHV Briggs & Stratton. I checked the hood and found nothing that would prevent proper air flow over the engine.