protein skimmer help

My first post so please bare with me...
My problem is that my Prizm Skimmer Deluxe (Red Sea) keeps stopping pumping air/water. I just installed the skimmer to my 75 gallon FOT over the weekend. I have followed the instructions for the setup of the skimmer. When I attach the air tube to the II slot (for pumping air and water) everything seems to work fine for awhile (the overflow compartment fills up and bubbles are produced). This is what is supposed to happen I believe. For some reason it stops pumping water in a few hours ir so. I then have to attach the airtube to the
I (#1) slot (for pumping water only) to get the skimmer pumping water again.


Any ideas why this is happening?

Am I explaining this right? What should I see happening? I am a newbie so please bare with me again.

AJ

As a matter of fact I had to turn the valve down at 1 AM because I did notice the large amount of small bubbles. It was running fine when the valve was open all the way. It stopped after a few hours when I turned the valve down.