Sounds like very low voltage from the battery. Bad ground cable from the battery or low charge/aged battery. Grounds are very important. Cranking voltage should never fall below nine volts. Make sure you have a good battery and grounds to frame and the engine block. I start out taking a digital voltmeter and putting one lead on negative lead of the battery (cables connected) and the other lead to the best metal ground on the engine and then the frame. You want to see the lowest 0.01 volt to 0.02 volt you can get. Any thing above this means less than a perfect ground. Cables can have corrosion under the insulation. Copper should look like copper. Not black colored or blue powder anywhere.

Most of the electronics operate on five volts and very low current draws. Connections of all types have to be clean and bright. The more new looking the better! I use electronic control cleaner on injector clips and simliar connectors. Just taking them off and on scrubs the surfaces that can temporary make things better! All these things have to communicate. At 5 volts its like smoke signals. Tarnish is like having fog!