Came across this discussion when I was in a state of desperation myself after 3 clutch masters and 3 slave cylinders. Also replaced most of the linkage too. Read here somewhere that 2a return springs at the slave can be a problem. I ran out and...
Alternatively iirc, it’s a bugger to nip up the collar on the trans. Make sure the threads are clean and it is done up all the way. Remove the trans speedo drive to ensure it’s blue as other colours are different gearing. Then you can inspect...
Potentially the plastic insert in the trans is worn or similar in the back of speedo. As pointed out driving with cluster out will rule out the former.