Fluctuating flue draft Miller CMF2 80 PO CONV

[ctwap2783]

Hello
I am in the weatherization field and part of that duty includes combustion appliance testing for safety. One of those test is checking the draft in the flue pipe. I use a Bacharach combustion analyzer and digital manometer to check this. Lately I have had numerous mobile homes each equipped with either a Miller CMF 80 or Miller CMF2 80 PO CONV unsteady fluctuating draft causing minor Back drafting for short periods of time. I had a HVAC tech look at one of the units and he said the heat exchanger was cracked so the client replaced the furnace. Unfortunately the new unit is doing the same thing. Has any experienced this problem or is this normal operation for these units?