Potion of Greater Invisibility Runs Out Fast

The potion of greater invis no longer lasts 10 minutes for me. For about 30 levels it worked fine but all of a sudden the potion runs out after 1-3 turns. I tried taking all my armor off to see if that was the cause, I tried using dispel and remove curse and nothing seems to fix it. I'm an elf wizard with 5 Theurg. Someone said using theurg may cancel the potion but for the 30 lvls I used the potion I could cast light dart and summon fairies with no issue. No message appears in the Room box when my potion wears off. Any help would be greatly appreciated, thanks!

I encountered the same problem with invulnerability potions. You can search for my post further down. Seems they activated Theurgism, so the time a potion lasts is related to the number of points you have in Theurgism.

Thanks Ana!

Last time I tested, my level 600 wiz with 6k health and 5 Theurgism skill drank a potion and it lasted about four combat rounds. Seems like it needs a buff. Might have been a shifting potion (not invis), but still.