dftravis

i have a d16z6 in my ek hatch and it was dying before from what seemed like no charge. i checked it out and saw the alternator belt was old, cracked, and loose. so i replaced it thinking it would solve the problem. at first i thought it did but when night came and had to drive with my lights on it did the same thing: gauges went dim, rpm gauge stopped working so i had to rev it up all the way and everything went back to normal. it does it every so often but only when i have lights on and using more electricity. anyone have any ideas? also this happened after installing a new ecu in which my speedometer also stopped working soon after.could that be a problem? i could use any help or tips anyone can give, thanks.