Have a carrier weathermaker 90% HIGH EFFICINCY FURNACE ,this a spuratic problem.Sometimes when the stat calls for heat I can here the vent van come on and sometimes will run for 15-20 minutes, then the furnace fires up.some times it won't fire at all, shuting off the power switch on the side of the furnace for a couple of minutes, then turning it back on will cause the blower fan to come on for a minute or two, then the vent fan comes on and the furnace will ussualy fire up normally. Then it may work fine for four or five days then it will light hit or miss. I have replaced the ignitor 3 weeks ago.
Thanks for any help or suggestions.

Sounds like a intermittent combustion fan proving switch.