P0884 code

A few days ago a bunch of the dash lights came on and the speedometer went down to 0, the car lurched but never completely lost power. Tested the battery and replaced it, also tested the alternator - 14.3 volts. Replaced the serpentine belt as well. The car drove fine for a few days then today the exact same thing happened except the lights all went off and just the check engine light stayed on. Ran diagnosis and code P0884 came up. Sounds like the TCM is only getting intermittent power. What is the best plan for trying to figure out the source of the power loss?