Quick check/review of operation:

Normal operation is Heating only from the floor outlets, Cooling air only from the center outlets. Side dash outlets heat and cool.

Have you tested for floor outlet vacuum in Heating, #3 valve of the seven valves located to the right of the console?

Here are a couple of related threads, they might help

Heat/no heat issues

w126 heater vent intermitent by feet

Re-soldering the ACCU has helped, once other causes have been eliminated.

Best Regards,
Jim