I was thinking when I glanced back at this thread thinking wait it's summer so "cold" is not really that cold ie not cold enough to actually need the glow plugs. Right now mine are disconnected and I have no problems starting.
So now with that revelation, (don't ask why it took so long to come about) the next likely suspects are the batteries or starter getting weak.
Since diesel engines are compression engines and the heat for initial starting is generated by the compression of the air and assisted by the glow plugs. With the engine temperature greater than 60°F there is no need for glow plugs. I will say that I recently finished chasing similar problems. I know I had newer glow plugs, and a stancor contactor replacing my gpr, and my UVC harnesses & VC's were good and/or replaced when I did the GPs I had replaced the batteries about 3 years ago. So the last avenue was the starter to which I about choked when the parts guy told me the core charge was $10 more than the starter. So to end the long story after replacing the starter I am amazed a the difference in how the truck starts to the point now that I've disabled the GPs for the summer. (11&1/2 months a year)