Amp Meter Variation

I have an aftermarket amp meter on my 69 RR, and it used to work fine. But ever since I changed the cyl heads to Aluminum ones, the gauge acts strange. When the engine revs past 1500 to 2000 RPM, the meter goes to pretty much straight up. But when the RPMs come down towards idle, it goes to about 15 amps of charge???? Then when it was revved up, I turned on the lights and it went from straight up to a slight discharge. I put in a new voltage reg, and no change. Then I put in a better battery, and it doesn't over charge as much now, but still does at just above idle. The old battery had a bad cell that was allowing an over charge condition all the time. So any ideas? Could the alternator field wires be on backwards? Or maybe the alt is going bad. Any ideas will be helpful. Thanks.

Alghough something needs to be checked up, try to understand what the ammeter readings mean

http://www.dodgecharger.com/forum/index.php/topic,33574.0/all.html

And need to know stock alts tipically doesn't charge while iddling

However 1500 to 2000 is way too much. Usually 900-1000 is quite enough to get a nice charge output if required under regular conditions

Make sure your grounding is good. Do you have a ground from the head or block to the body?

And be careful, if the ammeter is spiking that high there could be some melting going on.

Also check the wire on the back of the alternator to be sure it has a clean connection.

P.S.
You should put a volt meter on it so see what your voltage reading is. I can tell you that the ECU's (if so equipped) do not like to be over charged.