Paul Slocum

Recap:
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The other day the car started fine, we drove about 20 minutes, and then
electrical devices started failing: first the speedometer, then the air
conditioner fan, the windows were slow, and after another 15 minutes
the car died (while driving on the highway) and the battery was too low
to start. There had been no previous problems.
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We got the car back from the dealership after replacing the battery.
They said the battery wasn't putting out enough current, but the
alternator tested okay. At first it ran fine, but now it's having
problems again. It's been hesistating when stepping on the gas, the
windows have been going slow sometimes, and the check engine light came
on. The problems are intermittant though. My GF also said that it
seems to behave better when the AC is off (note: she didn't test the
fan and compressor separately).

Any ideas? I'd like to have a better idea of what could be wrong
before we take it to the dealer again, especially since the problems
are intermittant, and they obviously didn't find the real problem the
first time. Does this still sound like it could be the alternator
belt?

thanks in advance for any help,
-paul