Same problem with a Mini-PCI 2230 card mounted into a new Intel NUC i5 model. It had all the characteristics of being an apparent hardware problem.
However, strangely, completely removing the driver (device manager, selecting the card, pressing delete button and checking to remove the driver software) and reinstalling Intel's complete software package (didn't choose driver-only) solved the problem, it now seems stable even in Wireless-N. Previously it could only be made stable by disabling N mode.
Edit: Scrap that, it didn't become stable at all. Just a random incident, also the working under G mode part. The card just plain and simple does not seem to work very well. Think I'll just flush this card down the toilet and get a $5 Atheros off Ebay which for sure should save me a lot of unecesary work.
Intel should simply stop selling these, considering how widespread and severe the problems seem to be.