Howard,
I had a very similar (if not the same) problem in the Mk3 Gti I just got rid of.
After about 15mins when the car had warmed up it was very difficult to get it into any gear without a lot of 'persuading'. This was at a 98,000 miles, so count yourself lucky you got to 140k!!!
Like you I hoped it would be a simple linkage or bush problem, but local VW specialist said the diff was on its way out so needed gearbox replaced!

Apparently (according to VW specialist) "the diff gets tiny hairline cracks all over the inside of it, which allows it to expand when hot, making gear selection tricky". Not sure about the technical side of this, not being a gearbox expert!
I had a 'recon' 2nd-hand unit put in for ?400 all in. Just double check you get the right one put in as a Mk2 16V box will go into a Mk3 Gti 8V no probs! Car will drive like sh1t off a shovel around town, but will give you higher revs at motorway speeds as shorter geared!

Hope oyu sort yours out.
Scott