What would cause the following symptoms in an M46?

Sometimes, the amount of effort that is required to select a gear is so great that I have to grab the stick with both hands and "slam" it into gear. This could be any gear-the problem is not isolated to 1st or reverse, which rules out the reverse detent plate in my mind.

Sometimes, it shifts like new. It might shift perfectly for days at a time, but eventually the gears become almost impossible to select. Letting the car sit overnight seems to help.

I've changed the transmission fluid. Didn't help.

Could this be a failing release bearing? Check the cable tension and it can't be adjusted anymore. This would seem to indicate that the clutch is worn, but it doesn't slip or exhibit other typical symptoms of a 'bad' clutch.

Could it be an internal tranny problem?

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Sometimes, it shifts like new. It might shift perfectly for days at a time, but eventually the gears become almost impossible to select. Letting the car sit overnight seems to help. "
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have you checked the condition of the tranny mount and the engine mounts?

before you start getting into the harder stuff, check them for sagging...esp the tranny mt. cheap and easy to change.

On my M47 I had the bolt fall out that holds the pivot for the release fork in place and it caused similar symptoms.

If you are good at matching engine speeds for the gear to be selected. Then shift without using the clutch. If this can be repeated without your "both hands and slam it into gear" method. It sounds like a clutch that is not releasing.
If you have no experience with clutchless shifts, then I probably will have to suggest not to try it.

If the engagement point is the same under both conditions ( pedal height at which the car begins to move) then I would suspect that the friction disc is hanging up on the splines of the input shaft. They get a kind of fretting corrosion on them sometimes, which allows the disc to rub the flywheel even after the pressure plate is pulled back. Not enough to move the car, just enough to drag on the trans to prevent easy shifting into gear. The final check is to see if reverse can be engaged without grinding ( depress pedal, wait a few seconds, then try) when it is tough to get into forward gears.
If reverse can be engaged easily at all times, then the shift linkage is at fault - but it seems from your description it isn't this. However you make no mention of grinding into reverse.