2000 dodge 2500 has 285000 miles and when it sits for a while and I go to turn it on and try to move it won't even if I mash it. But if I let the truck warm up it has no problems. Trans fluid is nice and red. Just curious of what ya think it would be

 

I would start with changing the fluid and filters. Make sure the new filter o-ring is seated properly. If this doesn't fix it, it could be a front pump issue.

 

Leaking piston seal.