If it's really cooling a little bit and you're not just feeling it's cooler because of the moving air, then the refrigerant charge is probably just a little low. Hopefully, the car has not been converted to R134a.
WHATEVER you do, don't let some bonehead talk you into having the conversion done. And don't try to mix refrigenants. That's a good way to destroy a compressor.
There are several websites where you can get a license to recharge with the original R-12 refrigerant. It will only cost you $10 or $15 for the license and you'll learn more than you ever wanted to know about your AC. You'll also be able to buy a can of R-12 from the local parts guy and top up your system, just like the old days.
Here's one site
EPA Test . Printable study guide is free, the test & cert is $15. I've heard of others for less. Well worth it IMNSHO.