Chris' advice is good, but there's about a 99% chance that your R-134 is low because you have a leak in the evaporator coil. Almost everyone on this board who has experienced the problem reports temporary success in recharging the system, either professionally, or DIY. (Yes, the pro way is best, but you can restore very good function yourself). Unfortunately, the evaporator leak will get worse, not better, and someday you will be faced with the major expense of replacing it (<$1000).

Gerald C