rich99ss

Hi im new to this forum. Ive been trying to figure out whats wrong with my car. I have a 99 ss camaro with a 6 speed in it and recently my speedometer has only been working intermittently. When im driving it will read 0 mph then go up to my speed for a couple seconds, flicker, then go back down to 0. Ever since this has started to happen the car runs like crap, it stumbles at idle, idles real low sometimes, and when im driving sometimes it will feel like its dying so ill give it gas and it will stumble and then take off . So i bought a speed sensor and installed it with no luck. it didnt work at all... so i put the original one back in and it worked intermittently again. I thought i was making progress at this point. Ok so my friend has a 00 z28 6 speed so i figured i could try his speed sensor before i coughed up another 70 bucks for another sensor. Well i put his vss in and it was doing the same thing... I dont know what else to do. Any advice?