Air flow thru vents

Have 2008 Toyota Tundra air flow thru vents was never very strong ( has dial knob) . Last night after I was at idle air flow was getting lower then higher then low again almost shut down then blasted the best it ever has ( only owned for a year appox. Then it went back to the was it was. On high it seems like it on number 2 if I had four speeds.

 

Have you checked the cabin filter?

 

ditto. that thing may be crusty!

 

New something is going on with flow can hear fan work well then slows down almost shut off

 

If it were an old Toyota, I would say that sounds like a failing blower motor resistor to me. Back in the day, you would start losing speeds one after another until your blower motor didn't run anymore. The late model Toyotas have electronic boxes that set fan speed, but you could still have a box that's headed south. You need access to a 2008 Repair Manual, or maybe @Roman can help you out.