Help with a voltage problem?

Got a 97 s10 with the 2.2L. Problem is that the volts drop when there is a load on the batteries( 3 yellow top optimas). When it's at idle it's fine but as soon as I start driving it the voltage drops. Had the batteries and alternator checked at Autozone and they checked out ok. Any ideas what to check next? I have a high output alternator but don't remember the amp

have you checked your ground(s) ? the fewer the grounds, the less voltage drop you will have

i have checked the grounds on the batteries and they are all tight.