egg-shaped rear wheel!
i went over a few potholes at high speed the other day, and i've noticed it's bucked my rear wheel. theres not a great deal of movement from side to side... but through a rotation, the tyre moves closer to the frame, then back again, whilst staying in an almost straight line. it's as if different sections of the wheel have different radiuses.
is this common in mid range mountain bike? the rims are Sun - Black Eye
is this something i can fix with a wheel truing tool? i've only trued a wheel once before, and that one was moving from side to side.
It's simple to radially true a wheel. Here's a guide: http://www.parktool.com/repair/readhowto.asp?id=81
Similar to doing axial truing, but you tighten/loosen spokes in pairs. In spots where the wheel is too high/far away from the hub, tighten two opposite adjacent spokes in that area. By doing opposite spokes, you maintain the same lateral position, while pulling the rim down towards the hub. Similarly, in areas where the rim is too low/close to the hub, you loosen two adjacent spokes to move it away from the hub. Some lateral-truing will be needed at the end because you really don't need equal left-right tightening/loosening due to the dish.
In your case, you may find that the low spots cannot be removed by loosening the spokes in that area. The rim has been bent inwards and has a permanent dent. If it's a low-end rim on a low-end bike, you can just loosen ALL the spokes in that area, about 6-8 of them. Then slide a 2x4 in between the spokes and whack the rim away from the hub. Or lay the 2x4 on the rim itself and whack the 2x4 with a hammer.