Riverview is only 5-10 miles or so of three towers on the east site. Your setup may be "overkill" and the east site traffic is coming in so strong that it's overloading the scanner and interfering with weaker signals coming from the west site. I know one of the SLERS towers is near Riverview. You may also have cell towers in the vicinity. You also have the television/radio farm in Riverview that will cause interference, even though it's not on the 800mhz band.
If you had a yagi, you could try pointing it towards downtown Tampa as that's the closest tower that carries the west site traffic. You also might want to try experimenting with your scanner's attenuator function or use an external attenuator or even a band pass filter, depending on the source of any possible interference.
Sometimes having a really decent antenna setup is overkill. I do installations in various cities and sometimes, a rubber duck in the building out performs a discone or yagi on the roof unless I introduce band pass filters or attenuator to the setup. Just wait until everything is on the P25 simulcast system. THAT will be a challenge to receive for many people.