I have a 2001 2500HD Silverado with 8.1 litre engine and Allison transmission with 25K on it. The transmission was serviced in October and found to be as good as new. I pull a 25 foot trailer with no strain. During my last outing I had stopped for awhile and when leaving the transmission would not come out of third and the service engine light was on. I stopped, turned the engine off and when starting again the transmission shifted normally. Coming home after driving 125 miles which included two steep climbs the engine light came on a few blocks from home and the transmission would not shift out of low when the light changed. I stopped a block up at the next light, turned the engine off and when I started again it would not come out of neutral. I turned it off, waited and started it up. The engine light was now off and the truck shifted like normal. Once the trailer was disconnected the transmission performed just like new. The codes found later were PO700-Trnasmission Control System Malfunction and PO778 Pressure Control Solenoid B Malfunction. I bought a Chilton book but these codes were not listed. It has only happened when the trailer was connected and when it corrects it works perfectly. Can this be related to any electrical feedback from my trailer? Or should I just go ahead and replace the Solenoid? Is it internal or external to the transmission. Do solenoid have intermitten problems? Thanks in advance for any advice you may have.