Right, most (not all) slopes, hills or inclines; any mobs or I want to run up often stop us in our tracks. We are left with our little animated legs left pumping away while we get nowhere - sometimes we are released after a few seconds only to be caught again a bit further on or, at in the case of mobs, they magically appear where the AI presumably assumes they should now be.
Now for me, a few quick jumps can get me over the invisible block just fine and a stuck mob does have it's tactical advantages (pincushion). Still, it's a niggling annoyance. My PC isn't exactly state of the art but everything else seems to run quite smoothly at high settings and lowering the settings to medium or lower doesn't seem to make any difference to this ground clipping.
Having to restart the game after adjusting further graphics settings for testing is driving me nuts. If anyone has encountered this problem and has a suggestion or knows the cause, please let me know.

See the suggestions in the topic Can't run up any sort of incline, just walk.