The clutch coil in the PTO may be going bad. When working properly, they require only around 3 amp's to stay engaged. The coil can start to short out, but still works. A higher current draw can easily overwhelm the low amperage charging system on these tractors. It may cause the voltage to drop too low for the ignition modules to work properly. (But not kill the battery)
If there is no mechanical drag from the mower deck, and the tractor has good power to move itself, or tow. Then it may be time for a new PTO assy. Try engaging the PTO with the belt (to the deck) removed. If there is no change in the engine speed, the problem is mechanical. (Excessive drag) If it does slow, or stall. It is the PTO.