I've seen this be a clogged heater core. Especially if the prior owner didn't change the coolant regularly or used stop leak. How was the system flushed? Just by adding flushing compound, or using a garden hose at the heater core?

How was the coolant bled? Could be trapped air in the system.

Less likely, but could be the blend door sticking. Is it always the same temperature, and if it's dual climate control, is it different on each side?