Howdy
It sounds like the glow circuit. I would check the relay and associated wire connections first. I believe the light on the dash is a direct connection to the glow circuit. In other words the light will be dim if it is not getting good voltage to the glow circuit. A relay can be a fickle thing and could have sporadic symptoms for a long time before coming to a complete failure. Also a poor signal connection can be giving unreliable performance.
I had a very similar problem with my 1996. pulled the relay, benched it and it failed 8 out of ten cycles and then started working again and I couldn't get it to fail. Put in a new one and cleaned all the connections good and its been gold ever since.
Good Luck!
1996 Chvy 3500 CCDWLB 6.5 Heathed, NO vac, marine injctrs, ARB bumper, BIG pipe, 3" lift, bright lights, bypass oil system.
1986 Chvy 6.2 M1009 blazer RAM AIR, Headers, Custom interior
2001 Chinook RV, V10 gas
1974 John Deere 1530 diesel tractor
1993 John Deere 455 Diesel lawn mower
1967 GTO, 1989 Honda Transalp
2009 VW Jetta TDI, flashed and piped, 6speed, fun car!
1998 6.5 suburban, stock, daughter's
1993 6.5 3500 CCSWLB GM8, Heathed, big exhaust, gauges, Son's
1984 6.2 ATS turbo 3500, SCDWLB - Son's
3 kids, 1 wife, 1 dog
Gunsmith, Tactics Instructor, Fabricator USMC 87-93 Semper Fi!