Batboard

I have a GP350 UHF with 8 microvolt sensitivity. I have changed both ceramic filters, both crystal filters, and the front end module. I have checked volts to the module. I have checked resistance to ground at antenna contact, antenna diode, input and output of module. What do I check or change next?

If you have access to an RF millivoltmeter, you could check the synthesizer 1st L.O. level at the first mixer real quick, and if that's OK, check the 2nd L.O. level and frequency at the IF/demod IC.
If everything looks OK, and assuming you have a schematic from the service manual handy, inject a known level on channel signal into the antenna port and trace levels through the front end, first mixer, etc. until the levels don't approximate what's shown on the service manual schematic.

I have found that the problem is in the mixer or oscillator injection. These areas are not serviceable.