Also check whether the system is Simulcast over a number of sites, or whether your receiving a standalone site.
In Simulcast mode, two or more towers transmit the same information over the same frequencies at the same time. If you are in the 'overlap zone' in between two towers you might be experiencing 'multipath', where both towers are cancelling eachother out.
On an 800mhz system this area is quite small but its something worth checking.