Speedo Question

I have a 73z that the speedo has started playing up on. It would increase in speed slower than you were accelerating, like it had a lag, so I pulled the cable out and gave it a bit of lubrication. When I put it back in the speedo would not work so I drove it with the cable sticking out under the dash to make sure it was spinning, it all seemed fine. I put it back into the back of the speedo and it appeared to work correctly upto about 40mph then it stuck there, even when backing down the driveway. After I stopped it slowly crept back down. The Odometer is functioning correctly and seems to be recording the correct distance travelled. I am running out of ideas, the only thing I can think of is that the part of the speedo that makes the needle has some sort of problem. Has anyone else struck this problem? Any other ideas?