Q: Power issues like battery or computer

Ok I use this van for my business one day last week I was working and sat for 3 hours didn't know the door was left open and the inside light stayed on when trying to leave had to get a jump started right up drive home no issues. Next day drive all day no issues stopped to top off the gas.. after getting gas the van again would not start ( only needed 10.00 worth to top off was almost full the whole time ). Got a jump found a loose cable tighten it went to a local major parts store they checked the charging system told me alternator was bad didn't test the battery cause couldn't jump it so took it to a shop they tested alt, started and battery said battery was bad alt and started tested good. I was happy cause cheaper they also replaced the + post end on the cable. Now when driving the van the charging on the dash moves up and down from 14.8 to 12.0 up and down and when it drops fast the van acts like it lost all connection tgecdash lights come on the gauges jump all the way over

My car has 166000 miles.
My car has an automatic transmission.