I would try and readjust the clutch again and see if that helps. According to the Haynes manual:
... there should be 1-2 milimeters of free play at the bell crank lower end.
That's not very much play.
Secondarily, I see from this forum, you might have an issue akin to one I had on my car way back when.
Check the firewall where the cable goes through. As he says his mechanic found the following:
The hole through the firewall where the clutch cable passes was torn in 2 places, nearly an inch long allowing to firewall to flex every time the clutch pedal was pushed in. The fix was to install a plate, 1/8" thick over the broken hole, fasten it to the firewall and then put correct sized holes in the plate. It seems to be working perfectly now.
If this is not the case, I'd suggest the synchros in your transmission are going out and you'll need to look at a rebuild/replacement.