Where is your speedometer? Left or right in the dash?
If it's left, you can get by with a stock-length VW sedan cable but it will have a tighter-radius bend.
If it's on the right, you'll need a slightly longer cable to fit without binding and that is part number113957801AG at www.bugcity.com
Usually, needle jumping in the speedo can be minimized by making a longer, gentler curve in the cable from the wheel to the dash (one long curve, rather than two shorter ones). A tighter bend will cause the cable to take a "set" of the bend when it is stopped (99.999% of it's life) and when the wheel turns that set will "pop" the cable in the curve on every rotation and THAT is what you see when the needle jumps.
Install a new cable, the longer the better, make the cable take one large radius from dash to front wheel and that should cure it. Lube it before you install it just like a clutch cable: Pull the cable out from the dash end, put a glob of Molybdenum grease in your palm, draw the inner cable through the grease and slide it back into the sheath, then install as per your service manual.