Could be poor pilot flame quality due to dirty/greasy buildup in pilot burner assembly or poor adjustment of gas supply feeding it.
Could be as gas pressure issue from the source, causing the main burner to starve the pilot’s gas supply. A weak pilot will eventually cause lock-out.
Could be a poor wire connection between the combination valve, thermopile, high limit thermostat or operator thermostat – causing a thermopile output voltage drop..
Could be degraded/failing contact points inside either of the two thermostats – causing a thermopile output voltage drop that’s leading to a lock-out.
Could be a weak high limit thermostat.
Could be a sticking operator thermostat, causing an overtemp condition that trips the high limit.
Could be that one of the thermostat’s gas-charged bulb and capillary has gotten beaten up and developed a very slow leak that hasn’t yet leaked enough of its charge to cause total failure yet.
Could be a sticking combination valve, causing the main burners to stay on until an overtemp condition occurs.
Could be a few other possibilities…
I hope this helps. Perhaps someone with a DC voltmeter, ohmmeter, manometer and an intimate knowledge of the system’s operation can narrow it down for you. If you’ve already performed necessary tests with that equipment and would share the results here, post the results here and maybe we can narrow it down to what’s causing the problem.