1995 K3500 speed/cruise problem

I have a 95 k3500 4x4 7.4 auto. My cruise is acting erratic. I also noticed my speedometer above 60 acts lazy. Sometimes I can let off the gas and it will start to climb before it drops. I have installed 2 aftermarket VSS. The first one would only read to 50mph and the second to 60mph. I put the old one back in. Does anyone have any ideas?? I am thinking of trying an OEM part but am not sure this is the problem.