If you have too many items on your wishlist for it to load, you can file a ticket and request that staff clear the wishlist for you.

I think the code has since been changed to not let you to add too many items to a single wishlist category. But if anyone has noticed this is not the case, please let me know. There should be a limit at 500 items now but there might be ways to get around it still that need to be fixed.