when your a/c shuts down, i assume it's still turned on - look and see if the compressor is still spinning (with engine still running)

if the compressor is not spinning (engine running, a/c switched on) try turning off a/c for 10 minutes, and then turn on again, and see of it works (compressor spinning) - if so, you could bridge the 2 terminals on the pressostat next time the problems occurs, to test the pressostat (this is switch on low pressure a/c line, next to where the a/c pipe comes out of the evaporator on the bulkhead/firewall)

Try searching for posts (my posts) on 'compressor' for more details

Kevin