terex tc50 hydraulic problem

I have Terex TC50 excavator, ser # 0219. machine operates & works good for 15 - 20 minutes until oil warms up to operating temp. It then starts to slow down operating speed and will quit operating or track motion. It acts like the pump is deadheading as it will drag the engine rpm's to a stall if hold a function for a moment. When the machine cools down it will work for 15 - 20 minutes and quit working again. Anyone have any ideas on what to look for? Thanks, Terry

terex tc50 hydraulic problem