Know a bit (very little bit) about hydraulics and I think your last statement needs further investigation and repair.

If the bucket is creeping up, then that means that most likely the valve that controls the bucket lift circuit is either not returning to "rest" when released, or has a bad seal on the pressure side that's allowing hydraulic oil to bypass.

The constant demand for pressurized oil will definitely result in a hydraulic system that is constantly working, and therefore placing a load on the pump, and the consequent heating of the oil as a result.

Combine the constant load of the pump on the engine, along with the heated oil and you've got a recipe for engine overheat.

Can you "giggle" the bucket lift handle and get it to stop creeping up? If so, then the valve is not returning to rest. If not, then I'd look at replacing/repairing the valve itself.

The bucket should definitely stay wherever you put it as long as the engine is running. It should not creep upwards. For one thing, it's not safe.

Hope this helps - good luck.