Battery light flashes under full throttle?

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I had the same problem with my 98 GT.
I replaced the tensioner thinking it was belt slip.... But that didnt fix the problem.

The problem was the Alternator starting to go out. I bought a new alternator and the issue is gone!!!
 
+1 also replaced tensioner, no joy. Turns out it was the voltage regulator inside the alternator. Got a new one from RockAuto for ~$130 and was good to go.

I think my alt is a 105A unit... I was having some issues with light dimming but I'm also running two JL amplifiers. A .5 farad stiffening cap fixed that problem.

But back to original point: yes, you highly likely need to replace your alternator. It will get worse. Mine started out only flickering the battery light at high rpms. Then it became intermittent for minutes at a time. Then it stopped charging all together and I was stranded.

Spend the coin and remove the 6 bolts and 1 nut that are involved in the install. Very easy.
 
Was the alternator on my car. It would come on for a little while under normal driving, turn off for a bit, come back on...etc. Was on my way to buy a new alternator and my battery gauge just started dropping. Ran a red light into the Autozone parking lot haha No issues after the replacement.
 
Some ppl have reported similar problems at high RPM's which has been traced to excessive brush bounce. The symptom only occurs at high RPM's. The solution is to install a slightly larger alternator pulley.