99 Astro

I have a 99 chevy astro and I have put my new alt on, the wire that connects to the back of the alt with the bolt was very short and difficult to reattach so I have used my own 10 guage wire to go from that bolt to the possitive on my battery under the hood, the reason I replaced the old alt is because it has always charged at 14.4 but it started getting really suggish on charging and would not go above 13.7 now that ive put the new alt on it does start off when I first turn it on at 14.4 but after I drive it for a while and turn my aftermarket stereo w/ amp and subs it drops but then when it goes to recharge it takes a very long time and does not get back to 14.4. I have 3 red top batteries and the 110 stock alt, all batteries test out ok even with a load, also when im siiting at idle and rev it up while its at 13.7 it does not increase my voltage, please help no one seems to have an answer for me