1995 Chevy Monte Carlo

With the light dimming as you drive the car shows that there is a regulator or altinator problem. Check the belts for tension. You can take it to a store like auto zone and have the altinator checked, they can also check the drain of the battery. When you let it sit for a couple of hours the battery will recharge some but will not do it for long unless properly charged. The speedometer and tach are electically controlled and will fluxuate when the voltage starts to get low. Check all the electrical connections at the battery to make sure they are clean and make good connections.