First thing that I would suggest is a factory reset. With the unit off, press and hold the chevron button while powering it on. It should display FP for a few seconds.
In choosing a different frequency, you need to know how the Xterra target bin segment widths are programmed relative to the various frequencies.
Target segment bin widths are narrower and the lower end of the detection scale and wider at the high end with low frequency coils The inverse is true with high frequency coils. So a silver quarter for example, will give a response that will neatly fit in a segment with a low frequency coil, but might give a response that is too big for one segment with a high frequency coil. The effect of having it be a bigger response for the bin width, is that it is likely to "bleed over" into an adjacent bin on either side, causing it to indicate a "jumpy" ID with multiple tones, making positive ID much more of a challenge (<-read "you'll dig more trash chasing that quarter"). MF coils have the most uniform bin widths across the entire detection range, making them the "most honest", and a good general purpose use frequency.
(check out the illustration below)
Now, understanding all of this, go read Randy Horton's four part blog article titled "Which Xterra Coil Is Best For Me".
Which X-TERRA coil is best for me? part 1
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-1
Which X-TERRA coil is best for me? part 2
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-2
Which X-TERRA coil is best for me? part 3
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-3
Which X-TERRA coil is best for me? part 4
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-4