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I also wonder what you mean by "far"? If images of the moon are ok, what are you viewing? Stars? Galaxies? Nebula?
The 7mm will give you 178x magnification in your scope, which isn't excessive, but it does require fairly steady seeing to view sharp images at that magnification. It's worth noting that the seeing in NSW and VIC has bordered on horrible for the last 2-3 weeks thanks to the jetstream.
I'd suggest the same as Steve - it has to be collimation and/or thermal equilibrium, in combination with seeing.
With lower power eyepieces you won't see the effect of bad seeing as readily. The higher magnification you go, the more seeing comes into consideration and if the seeing is bad (or your mirror is not ambient), the image won't come to snap focus - it appears blurry and no matter how hard you try you can't focus it properly.
Is that what you're experiencing?
I'd also suggest trying it during the day on a tree on a far off mountain or something.
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