OK I have scoured around for similar problems but because I'm unsure of what exactly is going on I'm all out of guesses.
I'll describe the background to this problem and I'm hoping someone with a clue will be able to point me in the right direction...
At the end of January I fried the clutch (exploded actually, don't ask..).
Got together another clutch, pressure plate and flywheel and with a friends help got it all in. The flywheel was fine but the pressure plate was looking pretty cooked so replaced the clutch and pressure plate, just in case.
All went well, took it for a test drive down the road, shifted OK at first then started slipping a bit in third and fourth. Third had always been a little delicate and had been advised there'd possibly be some clutch slip while it settled in. Got back popped the engine cover and noticed my friend had put the shift cable too close to the manifold and it had melted the sleeve down to the wire sleeve. This inner sleeve looked fine and I asked him if it'd affect the shift to which he said probably not. We were running out of time so I pulled it away from the manifold and wired it against the firewall until I could move it properly.
The problem got worse over time and even after moving them back to their original position it remains rooted.
I am thinking the cable is $$% and has been stretched or perhaps the shift assembly bent or something similar? I have take it apart and put back together now a few times trying to see what might be wrong with it but no dice.
Any ideas appreciated, TIA.