I came to the 705 from an old analog unit (instant feedback), so had to learn to slow down quite a bit. I also prefer the 3Khz coils, which I find require a slightly slower swing than my 18.75khz coil. If the numbers are jumping around, you are probably either swinging too fast, lifting up at the ends of your swing, or there is simply a lot of junk in the ground. You might also back down the sensitivity a bit and see if the ID becomes more stable; I often hunt at 27-30 sensitivity, then drop it to 20-22 after finding a target.
The 'jumping around' can be a pretty good indication that you are over some trash, as that's what will happen when you're hitting an irregular piece of metal like a ball of foil or a crushed can. In contrast, a solid, regularly shaped target like a coin will typically be a good hit at a very narrow ID range. (E.g., a quarter might bounce between 40-42 or 42-44, but that's the extent of it in typical conditions. A coin buried on edge may be more variable.)
If you switch over to All Metal mode and 99 tones, you can get more a feel of the target. For example, if you get a nice, solid hit, but the sound 'slides' off the edges of a target (kind of a scouping chirp sound up or down), there's a good chance it is trash. I hunt entirely in AM mode unless there is a lot of trash, just because you get more information out of the detector. Some people like it, other people don't -- I remember one person calling his 705 "R2D2" -- but it's easy to flip back and forth if desired.
When I hunt in the woods where there's not much of anything metalic in the soil, I actually stay in Prospecting mode -- definitely the deepest-seeking mode of the 705 -- until I get a hit, and then switch back to Coin & Treasure mode (all metal or discrim) to check whether the target sounds worthwhile.
Of course, the only way to know for sure what lies below is to dig every target. I dug a beautiful IH penny one time when I thought I was simply cleaning up more can-slaw from the park, it was such an awful and jumpy signal!
