Temp Control Cable/blend Door Issue

Hi. I have a 93 mustang with 37 k miles like new.
My temperature hot/cold knob has always been hard to turn and last week the plastic piece attached to the control cable broke.
I tried replacing the control cable but its only making my problem worse.
When I turn the knob to hot or cold it wont stay all the way and just seems to spring back.
The arm for the blend door moves freely but seems a little spongy when I push it down. I can hold it down with my finger but when I use the knob its like the knob is not strong enough to hold it down like the spring is too strong.
Ive tried adjusting the clip on the cable and its the same thing no matter where I set it.
Really dont know what to do at this point.

Thanks for responses guys.
I am using a brand new cable and I have this book.
The cable moves freely and the knob turns smooth until I hook it onto the arm at the blend door.
Like I mentioned I can easily move the arm at the blend door with my finger and I hear it opening and closing.
What is the purpose of the spring on the arm door?
I removed the spring thinking it was bad or too strong but same issue persists.
At times I can get it to turn smoothly to warm but same issue when I turn it to cold--it doesnt go all the way down and springs back a little bit. Like there is no positive stop on the cold side, feels like I can keep turning it till it breaks.
The gear tracks behind the knob are clean and clear of dust or debris.