Some findings after a bit of experimenting:
* Zooming way in to the problematic table revealed its TT font contents to be fully vectorized with no quality degradation whatsoever. This seems to be the major contributing factor to the problem.
* Using bitmapped fonts in the table helped, but of course degraded visuals and printing due lack of antialiasing. This lack of quality might be acceptable to some and not for others.
* Suddely for new projects the table, using TT fonts, now render with antialiased bitmapped letters that degrades fully zoomed, but quality is fully adequate compared to bitmapped fonts.
I have looked in both Excel and in Solidworks, but I haven't found any obvious settings that could influence this. So even if the problem have resolved itself, I'm still a bit curious to how that happened, for future problems of this kind.