1995 7.3, 108k miles. Has baby swamp injectors and a Hydra tuner, AFE big boost air filter. A couple days ago, it was about 20 degrees out and the truck started up as usual without being plugged in, 2 GP cycles. After sitting for 9 hours a work, no start. It would crank but not fire. Cycled glow plugs again, started to fire then just cranked. Repeat. Then it started and ran fine. Replaced the fuel filter, no gel in the filter. Put more white powerservice in, added some truck stop number 2 diesel. Same issue. Plugged it in for 2 hours, started right up. Glow plugs? Relay? Where would you start? This is my first diesel, learning as I go.