Problem with quantity discount pricing table.

I have just installed SP2 & 3 and seem to be having an issue with the quantity discount pricing table. I use "Price level B" for wholesale pricing in my store. Some items only get wholesale pricing if they are purchased as a case quantity, so using the quantity discount pricing table I put the quantity of the case and the price under "Level B." Here is where the problem is: when an item is entered for sale, with a quantity one greater then the case amount, (eg. case comes in 12 and a walk-in customer is purchasing 13) it will discount the item around 85% (not very good for my profit margin as you can imagine) as larger quantities are entered the discount amount changes till you get to double the case quantity (eg. 24) then it will use the standard retail price, then once you go greater then this (25) it will discount again. it seems to be very random on the discount %. Has anyone come across anything like this or know what's happening?