I mainly use CGCGCD and don't have a problem with the low C. I use D'Addario 13-56 strings (I have had a tuning problem with some other strings) and play with quite a low action withgout any buzzing. I would suggest that your guitar might need setting up by a good guitar repairer and maybe you should look at your technique - ensuring you play the strings downwards rather then at an angle which will snap them into the fingerboard. One word of warning - if you do put on heavier strings, make sure your guitar can take the extra tension if you tune back to standard tuning (please don't try tuning the bottom sting up to C again - your guitar neck won't like the strain).