my last films were pretty bad, i have problems with my new ring flash. and my scanner is dirty. and i have to try a little more with long time exposure.
do you make a point to bracket your exposures? like, take at least 3 shots of the same scene but with slightly different exposures -- for example, f/8 at 40 seconds, one minute, and one minute 30 seconds? plus, try films with a good reciprocity failure rating, like fuji's reala 100 or NPL (if you can find it anymore)