It sounds like you may be getting a low signal level which an outdoor or band specific antenna could help or you may be experiencing interference from something else that is overloading the front end and that can cause the control channel to drop.
The Uniden models usually do pretty good as far as rejecting interference from other signals though but all radio's are prone to what is call desense. GRE's are famous for that problem in the VHF high band mainly.
My guess is you are working with a weak signal. What does the little signal level bar graph show when you are or are not getting the control channel?
Also, make sure you don't have any priorities turned on that may be causing the 996 to tune away from the control channel while checking the priority channel or systems. I think there may be more than one type of priority check that can be enabled on the 996 so you may need to wade through the menu's and check everything if it is a priority problem. I think the Close Call feature can also run kind of like a priority so that would cause the radio to tune away from the control channel for a while.
It could also be the infamous multipath distortion problem that can affect all scanners and even commercial radios. This happens on sites that are simulcast sites meaning a system that has multiple towers all sending out the same signal. Your scanner can't deal with them as the signals hit your scanner a few milliseconds apart and that confuses the heck out of the scanner. The 996 will go back to just showing NFM when that happens as it is no longer decoding CC data. A loss of signal from a weal signal will also cause it to display NFM. There are ways to try and improve reception of simulcast systems that involve experimentation on your part. There is no one proven method that works for all. Sometimes using less of an antenna fixes it as the lesser antenna may prevent one of the towers signals from reaching your scanner. Some (myself included) have had luck using a Yagi (directional) antenna aimed at one tower only which causes the scanner to capture the signal from that one tower. Sometimes it is nearly impossible to fix. Some have better luck using a GRE scanner for simulcast sites.
The system I monitor works better on a Uniden for me. Some days my GRE will not pickup the system at all.
Post back your signal level first though. I'll see if I can find what state the county you mention is in as the database for the system will usually say if it is a simulcast site or not. Or if you get this, copy and past the link to the database page for that county system. I have no idea where Geauga County is located yet!