The voltage regulator has a built-in temperature compensation, at cold temps it will charge a a higher voltage, hi temps at a lower voltage.
That said, yours seems to be over-compensated (drops too low at higher temps), or you are not getting enuf airflow thru the alt. Check for belt slippage, fan too far away from the front housing, hot air entering the rear housing. An alt gets cooled by sucking air into the rear housing and out the fan.
If you have a one-wire, change to the 3-wire connection that has the voltage sense point at the battery.
If all that is good, I'd say the regulator is over compensated. Case in point, I had a Caddy Northstar in which I replaced the alt with with an AC Delco. Now it performs exactly as you describe......the issue is worse with the Northstar, the exhaust manifold is right next to the rear housing of the alt.
If you can, see if you can get a regulator from a different manufacturer, maybe even and old OEM part. In my case I just left it because the alt is such a b!tch to change...
george
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