GE furnice igniter glows as it should but doesn't ignite the flames what is wrong

I have a Honeywell RTH230B thermostat connected to my 4-wire system. I replaced my old Honeywell Mercury thermostat last fall. I just checked my wiring, and I have Green to 'G', Red to "Rh" (jumper is present), White to 'W', and Yellow to 'Y'. I used the furnace all winter without any...

When I start my furnace the burners stay on for 5 to 10 seconds then go out.I check the flame sensor its OK ?

Goodman GMPO75-3 ser9911605881 . When warmed up runs OK . Cold the DI fan runs then HSI glows,gas valve opens and it lights with woof woof and goes out in 5 secs . Cycles again and it might start or go on third try. I polished the fire rod, set GV to 3.5 ins then changed GV. 24 volts at the GV...

100K BTU ng,Honeywell 1982 furnace. New filters, good intake(I think). Has worked fine until last 2.5 weeks. Pilot light is fine, main gas ignites and will stay lit for about 20 seconds. The honeywell hi/lo limit switch clock turns to main fan, sometimes it will reach the fan on temp, but...