Andy, welcome. On the electrical side, if your alternator is good, then I'd look for a bad connection somewhere. Maybe the master ground on the engine has gotten corroded. Seems like a connection issue to me.
On the oil pressure, those readings sound about right. I don't know how accurate those gauges are. I have the Smart Craft monitor on my boat and it always shows a higher oil pressure than the analog gauge shows. They're reference at best. Mine drops a good deal at idle though so I think you're good.
O'
"Little Mascara 3"
2003 266BR,496 MAG w/Bravo III