but, being self contained, and more than one failing, tells me that its that time. I think that even IF you find someone that will do an exploratory inside the wall looksee to determine the problem, this will end up costing more than to replace the windows.
I know the type of window you are referring to and I don't know of any way to repair these windows without removing the window or taking out a section of the wall to check it out.
Save your money and budget a cause for window repairs.