Problems with a 1 month old MBA?

I bought a new Macbook Air from an Apple store 1 month and 1 week ago. The most intensive thing I have done with the laptop is have applications like Chrome, iCalender, and iTunes open at once.

3 weeks in, the MBA would randomly go to sleep. All my power saver / other system preferences are kept on default. This was not a big deal to me, since many MBA owners seem to have this software issue.

However, 1 month and a few days in, the MBA froze in between swiping between desktops and I had to force shut down. The applications I had open were iCal, iMail, and Chrome.

Yesterday, I had my MBA in sleep mode. When I opened the lid and got to the login screen, I typed in my password. The MBA froze and shut itself down. This happened again later in the evening.

I took the laptop to the genius bar, where they ran tests and told me nothing was wrong with my laptop. They were not able to replicate the problems I had in the store and were not able to fix anything. I had the option of leaving it at the store for 3-5 to run tests, but I am a full-time student and am not sure when the next time will be that can be without a laptop for that long.

The worker told me that the problem may be from me handling the laptop incorrectly. He suggested things like closing the lid of the laptop without having all the applications closed (I thought Mac's had the functionality of being able to close the lid on-the-go?).

Have any MBA owners had a similar situation before? Do you have any advice? I have not purchased the 3-year AppleCare warranty and am worried that I may encounter even more problems later on after the 1-year factory warrenty has expired.