flame cycles every 30 seconds

[mrbigtoughguy]

I have a Tempstar 5000
nuge125ak01
it uses a s8610u ignition module

I cannot find any "flame sensor" anywhere in the furnace

I have posted here before and we never really figured this out.
It seems like other people have this similar problem

I replaced the fan limit control and ignition module 2 months ago to try to fix this and other problems

my home is still well heated, but I am concerned that the constant 30 second cycle is going to burn out my ignition system and it is annoying

I tested the thermostat and it is working appropriately

I jumpered the flame roll out switch and it did not make any difference.

The only thing I have not tested is the pressure switch, which should not be affecting this problem

This is how things go
thermostat send heat demand
exhaust blower starts
lights flash on the ignition module (normal)
gas valve opens
sparker clicks
pilot light lights
sparker off
3 or 4 seconds the burners light
burners burn for 60-90 seconds
fan limit control dial turns until main fan starts
fan continues until heat demand is satisfied
PROBLEM AREA
after main fan starts burners burn for 30 seconds then they go out
immediately after the flame goes out (lights out on control module)
the control module lights up (normal initial operation) sparker sparks pilot light lights and burners ignite (all identical to initial operation and seemingly normal)
burners burn for 18-22 seconds and go out, back to start again.
this goes on over and over until heat demand is met.
it takes about 8 seconds to go through the ignition cycle and then 20 seconds of burn time. 30 second cycle.

I guess this could be normal operation, but seems like it will beat up my equipment too quickly

any ideas?

<-- that is what the burners do