A/C Only Intermittently Working

90 XJ...Sometimes my A/C is nice and cold, other times does not seem to "come on". Blower works fine, just doesn't cool. The only patterns I notice (and they aren't always true) are:

--when the A/C is on before I turn the key (and it happens to be a mild temp in the morning) it ususally stays on a while.
--seems to come on on long drives, once I have driven a while.
--sometimes if it doesn't come on, I can turn it off when I am driving and then back on and it kicks in.
--without knowing for sure, I guess I am thinking the coolant is okay because "sometimes" it gets very cool..I mean nice!

Any thoughts? I am pretty sure it hasn't been converted to 134a, based on the connections.

Here is a good A/C forum. http://acsource.net/acforum/index.php

You need to first check the charge level using a set of gauges. You might simply be low. Bad news, since your still on R-12, it will be expensive. You should call around and get an estimate from some local shops.

When was the last time your A/C was serviced... IF it has been quite a while... then you may be one of the many that have a system with a slow leak... This is called an acceptable loss (1-5% over several years).
Purchase 1 can of R-12 and then have a qualified tech or friend that knows what he is doing... slowly add the refrigerant to the system (ensure to use refrigerant gauge manifold) to verify pressures and superheat.