I've been putting in a lot of miles on my fixed-gear Raleigh since I built it up at Christmas, but I've noticed quite a severe tight spot on the chain. The chain pulls tight at one point, but then has about 1 cm of up-and-down slack in it after 1/2 a turn of the cranks. I've tried Sheldon Brown's trick of loosening the crank bolts and hitting the chain when it's at its tightest, but it doesn't seem to have helped, even though the bolts were completely loose. The cranks are some SR Suntour 4-arm ones, and the chainwheel is a 46-tooth Shimano one. The tight spot always comes at the same crank postion, so I think the problem's up front, but can anyone suggest how I might fix it?

Thanks in advance...