Charging system problem...

I have been having this problem for some time now. The car died on me. The alternator went. So I replaced the alternator. The battery and the serpentine belt.

The car was fine for a few weeks, then I was on the highway and the warning lights came on and the "service charging system" sign started flashing on the dash. Took it to the mechanic he looked it over SAID everything was fine replace the battery terminals. I drove it for about 20 miles, and it died again. Anyone experience similar problems? Anything I can do to diagnose this problem? Thanks!

And was the alt checked to see if it actually charging? When loaded what was it's output? What year is this, also left out.

The mechanic said he tested the alternator and it was fine. I don't know what it's output was when loaded... And it's a 97 Deville... Thanks.