http://mahen.free.fr/temp/fsuae50hz.JPG looks normal for full vsync
But "tearfree" option in Xorg.conf sounds suspicious, as that can imply some sort of buffering or interference with FS-UAE's attempts to synchronize with vblank. So I suggest you keep it off.
Another thing to note in the graphs - when full video sync is active, there's two colors in the emulation graph (yellow-ish and orange-ish). This allows you to see how much time is used emulating the frame (yellow), and how much time is spent waiting for vblank (orange).
You can see from the graph that sometimes, the yellow bit is quite high, which leaves little time left to transferring the texture to the GPU and render it to screen (with low latency vsync, this is done directly after the frame is ready from the emulation). So you probably have just a bit too slow system for low latency vsync without taking steps to increase emulation performance. See
http://fs-uae.net/performance for some alternatives
(Just FYI, FS-UAE itself never switches mode. It uses the current desktop resolution / refresh rate).
Btw, 49.8 fps is not necessarily a problem by itself. If your "50 Hz" refresh rate is actually 49.8 Hz, then that is fine, and FS-UAE will sync to that. If the OpenGL driver, the graphics card and the monitor all agree on the 49.8 Hz, everything is fine.
You can use the scrolling strip of triangles on the Ctrl+F10 overlay to visually gauge if the refresh rate is constant. When everything works properly, the triangle strip should never stutter (when syncing to vblank only).