Cat 252b Aux Hyd wont stay ingauged while running brush cutter

Have a cat 252b skidsteer with 2400 hours. Fresh Cat oil/ filters. Standard flow Hyd. Have owned the machine for several years. Never any issues with the Hyd system unless I am running a brush cutter. Currently running one with a flow requirement of 16-26gpm. Had the same issue with a cat brush cutter that I rented last year. The issue is that while the hyd continuous flow is engaged, the hyd flow will stop suddenly anytime there is any input in the RH joystick to roll the cutter back up. Sometimes I can adjust it as long as I go very slowly but usual will cause it to kick out. I can move the machine forward and backwards as fast as I want but seems like any movement in the bucker control will cause it to stop. Doesn't seem to matter if the machine is hot or cold. Not sure where to start?? The hydraulics seem to be very strong any other time I use it. Same problem occurs if you try to roll the thumb roller up to engage the hyd and manually hold it. will kick out and require me to restart the flow. It will do this sitting still with no load on the cutter. The cutter seems to have plenty of flow while it is operating. Any advice is appreciated.