HVAC not blowing through dash

i have a 96 pontiac transport and the hvac blower only blow through the floor vents and not through the dash vents or defrost. this seems to be an intermitent problem and is getting annoying. the fan blows good at all speeds and i am getting heat and A/C but only through the floor vent

This "looks" as if you have no action at all on all vacuum solenoids, like if the vacuum control box leaks or vacuum hose is disconnected or pinched somewhere. All vacuum actuators divert air to alternate outlets, but floor is "default value". In that case, you would not be able to use "RECIRC" (max) either: check for that when the problem is present.

I would not think the electric feed to each coil in the vacuum solenoid box to be in cause but check plug . A faulty ground - connector 6 - would do it all, but I think it is unlikely. A faulty/dirty contact in the control panel might be possible...but a faulty feed to ALL would also impair temperature control..but since you say you can choose warm or cold at will, this seems to disqualify that option...

www.avigex.ca/xport/hvac_delivery.jpg or
www.avigex.ca/xport/hvac_delivery.pdf

www.avigex.ca/xport/hvac_module_assembly.jpg
www.avigex.ca/xport/airflow_diagnose1.jpg
www.avigex.ca/xport/airflow_diagnose2.jpg
www.avigex.ca/xport/airflow_diagnose3.jpg
www.avigex.ca/xport/harnessconnectors.jpg

(Diagrams from the '93 GM service manual; this configuration covers from '90 to '96 with only panel face cosmetic variations)