ok so I'm new to the famous duramax allison combo. converted from a Ford man. I have a cc/sb 4x4 2500 single tow tune full exhaust and intake egr delete all the basics. roughly 150,000 miles and love the truck. my issue is this just recently it's started down shifting from 2nd to 1st extremely hard. it feels like it jumps into neutral and slams into 1st and sort of stalls and lunges forward upon stoping. doesn't matter if your in tow haul or not. empty or loaded it does it the same. I have a cel on and according to everyone who has pulled codes says nothing related to tranny. it only shows glow plug circuit fault for 3 and 5. trucks just been serviced fully serviced every filter changed and every ounce of fluid flushed. and still is have same issue. Danville performance did all my mods and dyno tuning