Hi Stephanie,

I am not 100% sure about these models, but a lot of speedo cables are square, when the male part has been worn down a little they can jump and skip in the female part.  As soon as they start to do this it tends to wear very quickly and the problem accentuates.

There are two ways to fix the problem, the first is obvious.  Just replace the cable, its not a momentous job, just a bit fiddly.

The second is to fit a bicycle speedo, and calibrate it to your wheel size.  But you will need to check the maximum speed before purchase as not all bicycle speedo's will read 100kmh or higher.

Cheers,

The 75 has an electronic speedo, and I think the GTV does as well because the gearbox is very similar.  I'd pull the gauge cluster out and check the connections/earths first.

Like Phil said the bicycle speedos aren't very good for cars, they don't record high enough speeds, and at the upper limit of their range they have coarse incriments, ie you're doing  around 100 and the speedo might flick between 92 and 108 for an example. 

Cheers, Scott