50-amp power supply
HI: Is there a schematic included with your power supply? Also, what is the brand and model number?
It sounds like a component is failing after heating up for 10-15 minutes. If it's a series pass-transistor type power supply, check the reference voltage supplied to the base of the pass transistor(s). Whatever is supplying the reference voltage may be the problem. Often, they use a small 1-amp regulator IC to supply the reference. If it goes bad, the reference voltage might be unsteady.
Is the voltage changing gradually, or just jumping around randomly?
Is the supply variable? It could be a noisy control pot.
I work with high-current power supplies quite a bit, as I am in the pipe organ biz.
Let me know if I can help.
Jason N-G