I have an 05 4500 Duramax/Allison. I use it to tow a pretty large horse trailer and I have about 17K miles on it now. 90% of those miles were towing. Yesterday I was just driving the truck into town with no trailer. At about 35 mph, the truck started to upshift and it like stalled for a couple seconds. It almost felt like the truck lost power, then the tranny seemed to grab and it took off. It felt really strange and has me worried now. Anyone else had this happen, or know what is going on?