1996 dodge cummins won't idle
 

I have a 1996 dodge Cummins. The truck won't idle, otherwise it runs great. Timing is 20 deg. ATS aurora 3000 turbo, ATS automatic trans. Pensacola diesel stage 1 injectors TST #5 fuel plate. The problem started about 4 months ago and is getting worse. I have tried changing the overflow valve and the speed sensor on the tranny, both suggested by friends, but no change. It does seem to be worse when completely warmed up and on warm days. Run fine when the weather is cool and/or the engine is cold. Any ideas.
Thanks