Common problem, I'm sure there're dozens of threads on this forum alone about it.
There's probably a large ampacity fault somewhere; the gauges and panel lights don't require much juice, when you hit the start circuit, which does require a lot, it pulls what's available to try and turn the starter. The fault then does or does not return to adequate capacity, which explains it's intermittent nature.
I always start at the primary, high amp side, beginning with the battery
, and check every connection for corrosion
and tightness, and every cable for uniform flexibility (uneven flexibility means internal corrosion). Sometimes connections can appear good, but if they've been in place for a long time, can fail to pass full current
. Best to remove, clean and reassemble everything possible, and use some sort of antioxidant 'grease' when reassembling...