hey i am jay

i have a Rheem Commercial Natural Gas water heater (Model # RFD 120-67-1)
i had problem with this water heater last year too. but after calling the service guy he just came and changed the thermocouple and it worked fine for like a month n turned of so i changed the thermocouple 4 to 5 times it worked fine till last month the water heater turned off on its own, and i called around to find out whats wrong with my water heater and i found i was using the wrong thermocouple. but now again the water heater started to turn off, even after changing the thermocouple and after it turns off i try to turn it back on the pilot lamp wont stay on, some times it will run for like 15 mins and turn off. but i did notice the gas valve was hot couple of times when i tried to turn the water heater on.. so is it a problem with the gas valve? please help me..
thank you