Now clean & tighten every OTHER terminal on the 2 heavy battery cables. There are ~7 total. Block, frame, & core support grounds; starter relay, starter solenoid, underhood fuse block...
When you lose power, don't touch or jarr anything, and test voltage across the battery POSTS (not the clamps around the posts). If it's normal, move one of the meter probes onto the clamp, note the reading, put it back on the post & move the other one to its clamp, & note the reading. If either is significantly lower, that's the dirty/loose clamp. If they're all about the same, move the (+) probe to the starter relay's front large post. If it's low, the (+) cable is bad. Put the (+) probe back on the battery post & move the (-) to the alternator case, or the green ground screw on the core support. If either reads low, the (-) cable is bad.
If either battery post clamp is heavily corroded, read these captions:
