Focussing depends very heavily on the lens... does the 18-55 have any problems focussing?
What kind of shutter speeds are you using with the 70-300mm? If its anywhere over 1/100 then your prone to camera shake which could be causing the out of focus look. Anywhere over 200mm really exaggerates camera shake so I would bet this is the problem.
If you don't think its this then i'd get the lens looked at. If its happening with the 18-55 then id get the camera looked at
