Alternator or is it something else?

The car: 1998 Isuzu Hombre XS, 60000 miles, 5sp, 2.2l.
Recently replaced alternator, battery and belt.
While the engine is cold (about the first five minutes) the voltage
indicator is 'unstable', as if the alternator is not working at all (approx. 12V) or is not putting out full voltage. However it will occasionally move up to full 14V. Even after the first 5 minutes the voltage is still dropping occasionally below the 14V. Also, if RPM goes above 3500, voltage will drop to 12V, no matter how long its been driving.
Could this be caused by anything but the alternator, or did I just get a bad new alternator?
Thanks in advance for your advice!
Hartmut
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Hartmut Engelke wrote:

Bad battery could cause problems. Voltage regulator could too but most cars these days have the regulator built into the alternator. Dunno what Isuzu does.
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Regulator is part of the alternator on this model. Since this is happening with a new battery it's probably still pointing to the alternator/regulator piece. I'll test the battery just to be sure.
Thanks for the reply!

(approx.
move
voltage
bad
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Hartmut Engelke wrote:

There's been quite a number of posts here regarding Isuzu alternator/regulator problems. Mostly having to do with voltage surges causing a computer problem which leads to a transmission that slips. Cured (in my case) by turning the engine off and on. Changing the battery somehow helped the transmission problem but did not cure it entirely. Previous owner had put in the wrong size battery in a vacation emergency out in the boondocks. Beginning to think it's a weak spot with Isuzu.
Frank
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I had a somewhat similar problem, turns out the belt to the alternator was slipping, after I figured that our, I tightened it, and the problem stuck around.. the belt was worn out.. put a new belt on it, and tightened it to spec and poof.. no more slipping.
Chuck
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Zex0s wrote:

The situation I speak of is a little different. You can be driving normally, come to a stop, then step on the gas and nothing happens except the engine revs up and the little "service the transmission" light comes on.
Frank
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wrote:>

The same thing would happen to me at times. Isuzu dealer never couls find problem. I finally found a very good mechanic to drove it home for ten days with all his meters hooked up to it. It turned out that it was the alternator. Replaced this and the problem is gone. I used to have to turn ignition off and then back on(for the last couple of years!!!).
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