Gas oven goes "Thup" every 5 to 30 minutes. What to do?
RESOLUTION: WEAK IGNITER, NO DANGER.

I called in an appliance repair person.  The "Thup" sound was just the gas coming on. (I watched the gas igniting and heard the thup at the same time, several times.)  The repair person said the igniter was weak.  It took over 60 seconds for the igniter to ignite the gas when the oven was first turned on. The weak igniter was hot enough to light the gas, but the weakness caused the gas to go off.  When the gas went off, it stayed off for a short time (perhaps 15 sec), after which the gas came on again and ignited. This on-off-on  happened every 30 sec or so when the oven was first turned on.  After the oven was hot, the on-off happened very infrequently.  Probably the oven was operating properly after it was hot, going off when the target temperature was reached and going on when the temp dropped below that target.  The appliance repair person said there was no safety issue but sooner or later the weak igniter would not ignite the gas.  In any case, a new igniter solved the problem.  With the new igniter, the gas lit 30 sec after the oven was turned on and the on-off-on cycling was gone: the gas just stayed lit.

I do not know why a weak igniter would cause the gas to go off after was lit.  I would think that a heat sensor would detect the flame.  The repair person said there was no separate heat sensor.  So, it seems the igniter serves as the heat sensor also.