Is there a particular product that this is occurring for? I went to the item you inked to and was able to change the options and add the item to the cart without issue. When the page is checking the available inventory it has to look this up from your db. Depending on the number of products and options that you have the amount of time that it takes to retrieve the information can increase. I did not experience any issues when testing the site just now, it seemed to be working just as it should.