Wont stay lit after 20 min with blower on

Question

i have a procom 3panel natural gas heater serial #3351383-- model mn
180hpa w/manual blower pf06-yjlf. when in operation and the
blower on it will run great for 20 to 25 mins and then shut down
includeing the pilot light-- if i burn it without the blower on it will
stay on great-- i checked the fan intake and it seems like it is
blowing out cool air and the sides of the heater get hot to the
touch--did my blower motor reverse rotation-- if so is a replacement
blower available