I've got a 2002 2500HD with the 8.1L and Allison 5 speed. 120,000mi. Original owner. Really love this truck and plan on keeping it forever! Heres the issue, a couple times in the last month I have started it up and let it warm up for 5-10 minutes(approx. 35-45 degrees outside in the driveway). I go put it in reverse and the gear indicator just blinks and doesn't shift. Try to put it drive and it clunks pretty hard and requires throttle to nudge. Put the transfer case in and out of neutral, let the truck roll etc. The first time it got over it, eventually, shifted, ran perfect. The second time I had to give up. Came out the next day, a little warmer out, didn't let it warm up but let the rpm drop to 750ish, slipped right into gear. Haven't had an issue for a week, ten days. Any ideas as to what could be going on. I'm no expert on the workings of the Allisons. Maybe a shifting solenoid, temp. related? Fluid in tranny is full. Truck isn't chipped, pretty stock other than exhaust and intake. Any help would be appreciated.