you are doing clouds? Thats odd that you get that problem. The presets render pretty nicely and they have a resolution of about the same and a render quality of 1.5 ish.
6 minutes a frame isn't that bad really, I have renders that take 20 -30 minutes a frame, I've hard frames take longer than an hour. It's going to be hard to cut that doen much more than 6 minutes what were you hoping for? I can't belive it's spotty with those settings. Try Mental ray that might fix it.
I took the cloud preset from the visor and changed it's resolution to similar rez but with the shading quality at 1.5, .02, adaptive/jitter, linear. and it took 2:20 on a dual g5 2.0 ghz, it looked a liitle grain like textured, I'm assuming that's what you mean spotty, so I uppded the quality to 4 and it took 5:17 and there is no spotting. can you post an image of your spotting, I know duncan understands taht a lot better than I do he might just see it and know right away what to do.
I also tried a mental ray test with the 4 quality settings and it looked the best and took 5:23 so give that a try.