There is a connection on the back of the control board that gives problems as you describe. Also there is a small potentiometer on the dial controls that contributes to the problem you describe.
Go to the section on this forum "TBS and How To Articles" and open it. Then scan down the page and find a Thread "Index of Crossfire problems and repairs". You will find a PDF with pictures that will correct your problem with the A/C.
Make yourself familiar with the "Index of Crossfire Problems and Repairs" It is a gold mine for DIY courtesy of forum member onehundred80.
And here is a link to the potentiometer fix : A/C potentiometer