is the compressor shutting down after a while because of to low or to of a pressure in the system? and possibly whe you turn it off and back on it is resetting and allowing the compressor to run again?

have you let the car run in the driveway with the AC on and watched the compressor for a while and see if it turns off after about 30 minutes?
if it does then turn the switch off and on and see if the compressor starts.

this might be a good start. I would also check the pressure in the system.

here is also a link for some Self Sealing Refrigerant that has a guarantee and may put off the evap change for a few more years. it cost 199.95 for R134 kit alot less than a 1000.00 evap change.

http://cryochem.com/index.html

hope this helps.

Dan

if you use the cyro-chem stuff let us know how it works.