T600 AC issue, clutch not engaged, then suddenly works.

I'm having similar problems with my 99 w900. Is your engine fan coming on automatically?

 

Check wire and connector at pump for a break. They vibrate and tend to break. Otherwise, either purchase a new relay or swap the relay in the fuse box.

 

It sounds like the fan isn't coming on when its supposed to, or the condenser fins have debris, that is reducing it's ability to condense while stationary. You should be able to unplug the fan solenoid to get the fan to run continuously and see if it cools down while sitting.

 

Only for troubleshooting:

Supply ac clutch with known good 12v, verify it isnt flaky

Check ac box when its on but not blowing cold, is there reduced airflow or is it just as strong but compressor refuses to cycle on? Low air flow indicates freeze switch is shutting ac off, but getting "stuck" in the off position here and there

Verify operating refrigerant pressures, could be a blockage on the high side or at the expansion valve which would read far too low on low, far too high on high side resulting in ac clutch turning off till enough high pressure is bled through the exp valve