I just resolved a problem with the ac working only 5 percent of the time, more often it worked at idle. I ended up solving the problem by removing the shim, replacing the low pressure switch, and I had a bad high pressure switch connector. If you pushed the connector down, the clutch engaged, but leaving it alone it was very intermitent. I resolved this problem by taking the connector apart (making sure to note which wire went where) and gently squeeze with pliers a little at a time while test fitting it.
If you want to check the low pressure switch, first, make sure that the system is properly charged with refrigerant. Disconnect the connector on the low pressure switch, and stick a short jumper wire across the connector. If the clutch engages then the low pressure switch is likely the problem. DO NOT drive it around like this.