1996 hyundai excavator pilot control plunger problem

I have a 1996 hyundai 200 lc. The pilot plungers that control the swing are sticking down. Not all the time, but when it happens it is scary. The strange thing is that it seems to work fine when I first operate it, but then after about 15 to 30 minutes it starts acting up. Took plungers out and inspected and cleaned them, everything looked fine. I don't know if old hydraulic fluid gets tacky when it warms up or if something else is causing it. But it isn't happening on any other controls

1996 hyundai excavator pilot control plunger problem