Such odd problems popping up 1.5.6 basic install

I have such an odd problem. I have a new(ish) installed 1.5.6. I have only used vqmod changes / extensions but most are disabled except the free export tool and a remove category count extension. And a css color change.

Here is how I run across the problem:

I add a product to the cart and view the cart.
I then click checkout and add my address info....

Here is where the odd thing happens, but only sometimes. If I then click the shopping cart link to go back to the cart I get a pop-up that just says "error". Nothing more.
I can generate this error about 40% of the time by clicking back and forth between "shopping cart" and "checkout" links.
No error in the logs and nothing more then just "error" in the pop-up.

What do you think it could be? Is my site so slow I am interrupting a javascript file and that is causing the "error" pop-up? (that is my best guess at this time).

Mike

After a clean install it still does happen with just the "error" pop-up. No log entry or anything else showing what it could be but I do know that if I wait for 10 seconds between clicking another link it never happens so it makes me thing maybe it is the javascript that isn't firing fast enough and give the "error" pop-up when it is interrupted.