So, there's an issue I've found a few times now...
You are close to your potion limit. You finish a quest. Time to loot! And as you loot, you start moving at infinite slowness because... You just hit your limit!
I'd like to suggest there being some kind of timer (say, 1 minute, say 5, say changing area) between reaching the maximum at your bag and being slowed to death. Just to make looting and such a little more convenient... And also, the brewing and the buying of large amounts of potions.
This "timer" could also be limited by another timer (say, 1 hour) to ensure people dont simply drop potions every 5 minutes to grab them again, if we want to avoid abuse... I just generally feel sorry for the looter, as they inevitably reach their max, struggling to set everything on the floor in an orderly manner.
Alternatively change the penalty associated with being over the cap to something that is still very undesirable but doesn't grind things to a crawl if you're in a safe area.
For example you could instead inflict -15 AC, -15 AB and Spell failure of 90%.
Anything less than a massive movement speed penalty and certain classes or PCs wouldn't have to care about it, such as invisible backliners.
Having the penalty amp up over 60 seconds or so to full might work though so you can still loot quest rewards for dividing.
Just remove the potion cap limit in quest areas only.