hydraulic issues

I have a Komatsu PC50MR-2 that works fine until the hydraulic oil heats up and loses viscosity (after about 30 minutes of work). It has an aftermarket thumb that is not operating properly, which I believe must be contributing to the problem. Unfortunately my machine is in a remote area and I can't find anyone to help troubleshoot the issue. Not sure how to isolate the problem...any ideas, anyone? I've changed hydraulic oil and filters, didn't help.

hydraulic issues