for what it's worth i have seen computers that for reasons known only to them, really don't like running as primaries..
please bear in mind there is NOTHING wrong with running as a secondary, it doesn't make you a leech, it doesn't mean you're not doing your bit to support the network.. sharing files with the higher bw available from not running as primary is just as much a "good thing"
in short, if your machine is happier running secondary.. then run secondary