AC Belt issue
 

I think my AC belt is either slipping or not tight enough. My AC cools but every now and then it won't cool and it takes me to go highway speeds for it to cool again, but won't do it at lower speeds or idle SOMETIMES. I hear it squeaking every now and then, but one time let out a big squeal as I was on the highway then the AC finally kicked in when it did. Does it need replacing? I mean, the belt was changed around not more than 60,000 miles ago. Or, is there a "simple" way to tighten it for a novice, weekend mechanic like me?

I was also thinking possibly it just needs a shot of AC lubricant or R134a with lubricant. I did that with my other car who had similar symptoms and after adding the R134a with lubricant the compressor starting kicking in more often. The only difference is that my other car hasn't squeaked like my XL7 does.

Any clues would be appreciated as always! Thanks in advance.