The Yaesu FT-847 is a fantasic rig but it suffers from frequency drift that is especially bad when the radio is operated on UHF SSB mode.
The problem is made worse when the radio is operated outdoors or in a mobile situation.
The culprit is the master timebase crystal X1001 that is located on the bottom circuit board between the plugin crystal filter modules. The fan in the front of the radio blows air that is recirculated in the radio directly onto the crystal. The heating and cooling effect from starting and stopping transmitting makes the frequency shift and rise and falls in ambient temperatures can make it worse..
The best way to stop it is to cut out a small block of styrofoam and push it over the X1001 crystal.