Bad/Good Voltage regulator

I have a 2000 Sienna LE 3.0 that needed a jump then was chk'd by ...zone & they said the voltage regulator was bad when they hooked up the checker(I saw the reading) and that the battery needed a charge. After the charge I went to ...boys for my lifetime warranty on the Alternator, bought a new one & paid my mechanic to install it, after 2 weeks this whole thing happened again exactly. After I paid my mechanic to install again, I brought it back to ...boys who then checked it and said it was good after 4 chks they almost didn't refund me ($250) but finally did. Symptoms were dead battery, A/C that changes speeds when stopped at light (slow speed) and when I take off goes to normal high setting I have it set at, weak sounding starts and electric windows. I get the feeling this is starting again, Help please.