Help - M 500 stalling out

[bobcatsam]

I have what i beleive is a late 70's early 80's M500 Bobcat with a 2 cylinder Kohler motor. The machine runs fine although after about 20-30 minutes of use the machine wants to stall out when any hydrolic command is beign used. If no hydrolics are being used it will keep on running. Turning the machine lifting something or tiliting the bucket stalls it out. Any ideas?