It's 33 years old. It's probably tired of working.
On the other hand, if it has a pilot light, that flame is suppose to keep the thermocouple hot, which through the magic of electronics (assuming your old tank has an electronic control) holds the propane valve open, and the pilot light lights the propane. Those thermocouples can go bad, or the connections to the thermocouple can corrode and cause it to work erratically.
In addition, not sure but you might have what's known as a thermal fuse inline with the 12 volt supply to your electronic control. Check that right off. They open circuit for no reason. They are available on eBay, $6 for 5 of them. There's usually a set of thermodisks too. Each one of those can go bad. All three of those devices can be checked with a simple ohmmeter. And they should register as a short or near to one. All testing can be done outside under the heaters cover.
To test either thermaldisks or thermal fuse, just short them out. One at a time.
That's if you have any of them. Not sure without the model and brand.
