Sounds like you could have an intermittent short when you have brief power to the instruments. Could still be the control module is faulty as it does contain circuit breakers,but I would still start at the basics! Either have the battery load tested or replace with a new one and see if you have power.If not then you need to start at the wiring and connectors to eliminate any shorts/bare wires,etc. It's just a process of elimination. Kind of a coincidence as to what just happened to me today. Turned on the kitchen light,it flashed once,turned the switch off and on,NO light. I just replaced all the breakers in the panel box 6 months ago,but checked to see if the breaker was tripping.It wasn't and had power.) Pulled the kitchen switch out,checked that it had power to the switch(yes, 120 volts) narrowed it down to the 31 year old single pole switch had shorted out. Replaced it for $1.50 and all's well in the world for me right now
Same thing you have to do on dc voltage,just start at the battery,connections,all the way up to the control module. Check that the speedo connector has at least one hot wire(memory) and that the ignition switch itself has power to one wire with the switch off. Check that there is power at the ecm harness. If there is and no power at the ignition switch or speedo harness,then it's probably a faulty ecm module,if you've eliminated any harness ground or short problem.