[hatzie]

A test light, while useful in some situations, will not give you the resolution necessary to find this intermittent.
You're either chasing a corroded or damaged connection or you have a bad internal connection in the battery.
This is tedious and time consuming to chase down but it ain't rocket science.

First off, Do a visual check for corrosion or damage. If a cable jacket is lumpy and not smooth the wire has corroded internally and is causing issues. Replace any internally corroded wires. If the jacket is damaged replace or repair the damaged wire and protect it from whatever damaged it.

Do a voltage drop test on each of the cables from the battery to the firewall bulkhead plug. You also need to do a voltage drop test on all of the ground cables and connections.

You need a decent Digital Volt Meter (DVM).

I intensely dislike cheap autoranging meters for voltage drop tests because, unless they are fairly high end (expensive), they tend to jump all over the place and not settle down soon enough to give a good read.

You can set the range on some inexpensive autoranging meters like the Extech 330 but your meter needs to have a range button to do it.

The meters I like to use on this kind of stuff are the old Dinosaurs or new $40 cheapies with several DCV settings say 200mv, 2v, 20v, 200v, 1000v and not just one setting for VDC.

Whatever you get for a meter get a decent set of probes. Look up Pomona 5519A. Those Pomona probes are generally around $20 but they will outlast several cheap meters. The probes that come with most meters, including my Fluke, are darn near worthless. The set that came with my Simpson 260-7 Analog meter had loose internal connections that made the readings float. The set that came with my Fluke 117 were no better.

Set the meter on 20VDC or whatever range is closest to but more than 14.5VDC and check the voltage between the lead posts on the battery. It should be 12.4-12.6v if it's in good shape and fully charged. You need the battery to be fully charged to run a voltage drop test so if it ain't fully charged put a charger on it and make it charged.

On a Top post battery check the voltage from the Neg post to the Pos battery cable lug and the same from the Pos post to the Neg cable lug. It shouldn't vary more than .01v. If it reads a difference of more than .03v clean the connections and re-test.

If I have the original side post battery I assume the connections are corroded and disassemble both Pos and Neg terminals... pull the bolts and strip off the non-molded protector shroud, clean all parts thoroughly with baking soda and brass brush and lots of water, and re-assemble with protectant.

Turn the ignition on and turn on the headlights.

Set the meter on 2VDC and probe from the Neg lug to the Ring terminal on the alternator bracket. There should be less than 0.03vdc.
Probe from the ring terminal to the ground bolt on the alternator bracket. It should be less than 0.01vdc.
Probe from the alternator bracket battery cable bolt to the intake. Again it should be less than 0.01vdc.
Probe from the intake to the firewall ground strap stud. It should be 0.01vdc or less.
If the voltage reads higher on any of these steps remove the connections, clean them, re-install with protectant and re-test. If they still read high replace the wire being tested.

Now start along the positive battery cable to the starter, from the starter to the J-studs on the firewall, from the J-studs to the Alternator BAT stud and to the bulkhead plug... You should see 0.03vdc or less on each step.
Again, If the voltage reads higher remove the connections, clean them, re-install with protectant and re-test. If they still read high replace the wire being tested.

Look the fat 12ga Red wires over inside the cab.
Are the 1/2" blade terminals on the ignition switch cruddy and burnt looking?
Do you have any obviously damaged wires at the fuse block, ignition switch, or the bulkhead plug. DO the terminals look overheated or corroded.
Fix whatever ain't right.

Some people allow up to 0.1vdc drop on each step but it's additive. Assuming the battery is 12.2vdc and the battery to cable and cable to starter and starter to J-Studs and J-Studs to bulkhead plug all have 0.1vdc drop the voltage at the bulkhead plug will be 11.8vdc and points inside the cab to things like the ECM will be even lower. You will get away with 11.8vdc but it erodes your safety margin and the corrosion or loose connections causing it will get worse over time. The ECM doesn't like low voltage.