You need a good starting point to find out what is wrong. It sounds like you have been arbitrarily adding refrigerant in an effort to make it work.

Recover the refrigerant in the system, replace the dryer, and check for leaks. When you are absolutely sure there are no leaks ( pull a vacuum on the system through your gages, then shut off the valves and leave the gages connected overnight, if the gage readings have changed other than a small allowance for temperature, you still have a leak) Charge the system with the proper amount of refrigerant as (you are using R12 right!) and then you can see if it is operating properly.

An undercharged or overcharged sytem will not cool properly, and if you have a leak, chances are there is moisture in the system, which can freeze in the expansion valve and cause intermittent cooling.

Once you have no leaks, no moisture, and the proper refrigerant charge you can start diagnosing any issues, if you still have any.