SEPTA never had any problems with them, and SEPTA had all 8 of the cars that came up to the LIRR. As many times as I rode them while they were here, only once did I encounter a warm car, and it was swapped out of the set within a day. It's not like they don't have occasional HVAC issues with the C3's (and they are not much to look at inside, either, IMO) or the MU's, especially the M3's. And perhaps the fact that they had to jury-rig the connections to the locomotives may have a slight unintended effect on things?
Humph. 1950's-vintage equipment, indeed? The MARC cars are at least two years newer than the M3's and have the same seats, albeit in different colors. Give them a bit of time, and they'll do just fine.
"CSX Detector. Milepost Six Point Four. No Defects. Repeat: No Defects. Total Axles Seven Four Seven. Detector Out"