Have you had anyone check the charge in it?
When the AC system is low on charge there is not enough "fluid" in it to draw away a significant amount of heat. So when it gets warmer, it can't perform. Since yours is about 7 years old it may have just lost enough due to seepage that it needs a top off. Since it still cools sometimes, most of the rest of the system should be fine. Most service facilities can properly charge it for you.
can I do it myself?
Those kits that you buy at the auto parts stores can do some damage if you put too much in. If you get one of those follow the instructions carefully and only add a small amount at a time. If under or over charge it can damage the system and get very costly. If you shop for coupons often service places will do it for a fairly low price, like Pep boys or tire dealers.