1995 Cutlass Supreme stalling/speedometer trouble

I've searched around to find someone with this same problem but havent been able to find it anywhere. The car I have is a 1995 Cutlass Supreme but I'm not sure of any engine size. The problem I'm having seems to be electrical and starts with the speedometer acting strange. While pressing the gas the speedometer will continue to climb faster than the car is actually traveling. At one time the readout was 90mph and I was only traveling about 25-30mph. When I press on the break the speedometer will spike for an instant and then drop right back down. While sitting at a stop sign it will bounce around from 5-10mph. Everything I do seems to make the speedometer fluctuate, even using the electric windows will cause it to bounce around. Also when the speedometer is messing up, the lights will dim or flicker with every push of the break. Rolling the windows up or down will also cause the same problem. While all of this is happening the radio, while being off, will make funny noises that are very loud and then the car will stall. Turning the car off and then back on will temporarly fix the problem, but given 10 minutes it will start acting up again. Does anyone know what could be causing this to happen?

I dont know if this information will help, but before this problem started to occur I had a new battery installed at autozone. The cap on the positive terminal was removed and the battery had slid over and touched another part of the car scraping of the paint and grounding out the car. When this happened, the car would not start for about 30 minutes but later started to work again. After that incident is when I started to notice these problems. Since this incident the battery has been strapped in and the cap has been replaced. So the battery is no longer touching the car.