I have a 2004 gmc 2500hd w/ 8.1 l and allison trans. Scanner says the trans is losing communication . While driving it intermittenly loses cominication causing spedo to quit, the eng light comes on, also the park brake light and abs light. This only happens periodiclly and only lasts for few seconds and back to normal. Have already changed ignition switch cuz all the components involved run through the switch. Still does it. Transmission modular?