I cant seem to find the last thread I put in, so I'm gonna try again!

I seem to have a "lazy" speedometer!
Every now and then it seems to drop to zero, and sudden wake up and tell me how fast I'm going.
I'm told it could be a stick cable? But I have no idea how to where to look, and how to fix it.
I'm a novice with the mechanics, even tho I am getting more involved. Last time, I changed the radiator. Not bad eh?

I have a few more problems with the car, like central locking not working, Windscreen demist not working, and a knocking under the front wheels. I'm sure there will be more unheard of problems with it coming soon. Last week the drivers seat broke, and had to get it welded.

PLEASE PLEASE HELP ME!!!