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Sounds like a weak thermopile. Unless this is wired to line voltage you have a thermopile not a thermocouple. The thermopile generates all the power (<1 volt) to operate the gas valve. They get weak over time and don't generate enough power to make the round trip to the thermostat and back. Replacing the thermopile is usually the fix for this problem.
A thermopile is just as easy to replace as a thermocouple. The issue is how difficult is the access on your unit. Usually it's not too bad since this is a fairly common maintenance item that does need to be replaced periodically. Thermopiles are universal with the only difference being the length of the lead. They are commonly available at any plumbing supply house and probably even the big box stores. You can confirm that you have one by checking to see if the flame sensor in the pilot flame has a small armored cable with 2 leads connected to the gas valve.
Like I said before thermopiles are universal. They haven't changed in over 60 years and everybody uses the same one. The only difference is the length of he leads and you can just coil it up if it's too long. You just need to confirm that you do indeed have one and I described how to do that in my last response. I should be around and I'll respond as soon as I can.