I've been having this problem since the 1.23a patch, and while I've found similar problems while searching (characters taking a long time to render), this seems to be different.

Characters, including PCs, NPCs, and mobs, will fail to load. In some cases, characters that had previously loaded will disappear (ex: I will have started a levequest with a party member, and half-way through the levequest, the character will disappear).

When characters disappear or fail to load, I cannot target them, so in the case where a party member disappears, I essentially become useless because I cannot heal them or attack the enemies that are attacking them.

The enemies and characters are shown on the mini-map, and visual effects and sound effects related to the characters render (footsteps, attack effects, etc), but nothing else.

This is frustrating, because while the problem only occurred every once in a while the other day, it has been persisting more and more every time I play. I used to be able to just log out and log back in to fix it, but now even sometimes that doesn't help. It has made doing any quests extremely frustrating because every time I visit a new NPC to advance the quest, I have to log out and log back in so they appear. Waiting in the area does not seem to make them load.

System specs:

AMD Phenom II X4 2.8GHz
8GB Ram
Asus 7750 1GB DDR5
Windows 7 64 bit

I had updated my graphics card recently, but I'm not sure if that's related to the problem, seeing as every other game I own works just fine.