Sounds odd.
You know your a/c works - just not all the time.
Could be something really simple... something not 'cutting-in' when it needs to everytime?
I'm sure an A/C specialist will be able to fault find better than a replace-and-try approach.
I asked Timpsons once to engrave a Cup for a School event, with... and I wrote it down for them, 'Stewart House'.
They did. Only problem was, it was engraved as, 'Stuart House'.
I rest my case.
A little bit. Small queues here for petrol ( but no diesel available. )
I reckon by Monday it will be old news.
I blame the media.
As long as we have Mince Pies for Christmas...
On a non Lexus I had the same issue.
It was the condenser - and stone damage I assume.
A new condenser fixed it.
I would take up the issue with the 'Lexus Specialist' you originally took it to, if you feel they are worthy of your trust ( and money ).