I recently picked up a '98 125 SX in excellent shape, but it had a very similiar bog to the one you describe. I used it as a bargaining chip, but knew it was a lean condition just off the bottom. Once the revs were up, it ripped. I had to turn the air screw in from 1 1/2 turns to 1/2 turn, which cleaned it right up. If we weren't so close to warm temps here in Maine, I would have put a larger pilot in it, but I'll just back the air screw out as temps rise. Hope this helps.
Dave