My 04 f350 6.0 wont start when motor is cold. If I plug block heater up it starts fine. All 8 glow plugs are brand new and all 8 wires are getting 12 volts. Any ideas??????

This all started with an oil change. Truck was fine before that. Since then, I've replaced rail pump, batteries, all 8 glow plugs, both fuel filters and oil filter, CSPS, ICP valve, fuel system relay, and put an O-ring kit on the injector closest to the bumper on driver side because when I pulled that glow plug it was very wet with oil. I checked EGR valve (looks good) I've cleaned all my grounds to ensure a good connection. The fuel tank is around 3/4 tank. My battery light has been flashing on and off as well. This is a new development but the starter still turns over strong. If the truck is warm or the block heater is plugged up it starts and runs great imediately but if its not plugged up and the motor itself has cooled down it is very hard to start and when it does I have to hold the accelorator to keep it running till it warms up and smooths out. This takes about 45 seconds then the truck runs like there's nothing wrong with it at all.