Laser Power varies

I'm a manager at a Makerspace and recently inherited responsibility for a donated laser cutter. It is a 50W unit very similar to the one linked below. We're using RDworks.

I replaced the power supply with the one linked below after the original one died. Now it seems to cut, but lose power after a while. I replaced the mirrors (2 of them had bad spots in them) and re-aligned the optical path. It did not come with an ammeter, so I installed one. The current aligns with the power setting (i.e. more power = more current) with a maximum of about 20mA.

Today I discovered it would cut 3mm plywood well at any location on the bed using 12mm/sec and 30% power. This used about 11mA. However, if you turned the power up to 80 or 90% it would not cut all the way through! It seems you have to get below 60% power before it will completely cut through 3mm, but if you go below 25% it also quits cutting through. Engraving seems to work fine and run all day.

Can anyone explain why it cuts better at lower power?
Also, what causes it to lose power after a while (sometimes it will work all morning, but not that afternoon, other times it loses power much quicker)

Thanks!

Similar Laser cutter: https://www.aliexpress.com/item/3294...AbTest=ae803_3

Replacement power supply: https://www.amazon.com/Cloudray-Engr...gateway&sr=8-4