Honestly, if it cools sometimes, and not others, I think that's a refrigerant issue. Mine's freezing my face of 90% of the time, but I do feel a spot every now and then that isn't as face-freezing as others. I really thing I'm a pound or so low on my refrigerant. What else could it be, if it cools massively the other 90% of the time?
I think it just doesn't have the refrigerant running through the compressor about 10% of the time. I think if I buy one of those $30 cans, add a little bit, it should take care of things (and I have a second car, same make and model, should attach and work there too.) I think one can will take care of both cars needing a little 'umph' after 15 years.