Engine john deere 160 starts to surge after 20 min and then stalls

I have a 1985 JD160 lawn tractor that will run for 20 min or so and then starts to surge (sort of like when a load is put on a generator). At first it keeps running and surging just slightly, but eventually it will stall out completely. When this happens I can get it started by putting the throttle in choke but to keep it running I have to move the throttle all the way down to full slow. This is happening wiith the plow blade on and the deck off. I have taken it in for repair once and they have not been able to fix it.