Climate control craziness!

Hello all, my '89 420 SEL is giving me climate fits!

In summer, AC works, but system switches from AC to heat and back to AC.
AC comes back on just before heat becomes uncomfortable.

In colder weather, heat comes on but AC cycles on and off as well.
Noticed that even when heat is on the vent that blows into the back seat blows cool air.

Exchanged climate control unit with no improvement.

Any ideas? I'm at a loss on this one....

Thanks in advance.