Toyota Avalon 1996

I am the original owner of a 1996 Avalon XLS which currently has 180,000 miles on it. Over the last tow years I have spent between 4-5K preparing the car to transition to my teens. The car has new rack & pinion, timing belts, hoses, alternator, battery, tires, CV boots, - I cant remember it all. Additionally, its been re-tinted and painted. Suddenly, in the last 3 weeks, the car is somehow draing the batter- completley! I'll run to the school, drop off a lunch, come back out immediately to find I need a jump. It has even died while idiling. My mechanic is persistent in trying to rule out how this is happening but so far no luck. The car had a new alternator for last fall that he replaced under warranty last week. He threw in a new battery even though he had no reason to suspect the older battery which was only a year old. Earlier this week, he replaced the ground wire and then connected the battery with copper (??). All that, and the car did it again last night. Failed to even start, had to be jumped, but then did something a bit new...it tried to die on me while driving it. The dash went dim, the headlights dulled. When at stop lights I had one foot on brake with the other on the accelerator trying to get home. I have loved my 96 Avalon and truly feel this is a 250-300K car or I wouldnt have spent all the money on it the last 2 years...I'm a bit confused by what is happening mow.