Speedometer issues

I have a 1991 chevy camaro Z28 5.7L l98 engine with 149,640 miles on it, the car has sit for 2 years and I just put all new gaskets and new oil pump and the basics, everything is running just fine, but my speedometer is not responding, at first I thought it might be sticking from sitting but when I rev the engine with it in park the speedometer raises to about 10mph and then slowly drops back to 0. When its in gear, when I first take off, the speedometer rolls all the way to the right and pegs then drops back to zero. I've checked the wire at the tailshaft and everything I can see seams fine. It sounds to me like a short somewhere but I don't know where to even look. PLEASE tell me you know what the problem is, it's driving me nuts.