Wish I was stronger at what Benzos were thinking with some set ups and features for you and some specifics but not so much.
Basics in that it can cool properly then quit like that.
I really doubt a belt and tensioner is it at all first off. It's either tight enough or not any you can tell that. It may need both but not consistent with the symptoms IMO.
Likes moving along so would be all over watching that fans are in fact working properly, possibly two speeds and you don't get the higher?
Dirty condenser and pressures might spike and shut down when stressed as mentioned - low speeds = less air flow naturally.
Other is gap on compressor's clutch. Just said that with another here and always a chance the gap is too large and heat matters so easy enough to check. .020 air gap. By .030 can be erratic or totally quit yet have power to it. Those if caught you should be able to see center hub not turning and tap on that while A/C should be on and if it click right on gap is probably excessive.
Another other: Is this converted to R-134a or still OE R-12? The total job on conversions meant adding a high pressure cut out if many R-12 OE systems didn't use.
If nothing obvious will really help to be ready with test equipment, mostly electrical test light should do that power to clutch oil at compressor is in fact empowered or not while down. All things will test fine when working fine.
You should have a sight glass on this too. If R-12 that's valid info of the state of charge which could still work good and be too low or somehow too high as well.
More info and let's see what we can figure out for you,