Please disregard anything that was mentioned above regarding Freon issues. You are not qualified to make sealed system repairs and the problem is not related to your Freon charge, most likely.
By your first description, you have over-cooling issues in the FF section. More than likely this is a damper issue or a runaway temperature control. On units with an adaptive defrost system, the board may be going out, causing the unit not to defrost as it should, when it should..
Have a look at the Frigidaire manual for side by sides, this one for units made in 1999-2001. It might help you understand the problem. If I had the year your model was made, I could provide a better manual.Manual