Sitbone Issues

[Paul]

Hi All,

I've been bicycle commuting for about 1.5 years and two months ago
purchased a Specialized TriCross for riding to work and road rides.
I've gone through many saddles and so far the best one is the
Specialized Alias. For the past 3 months my sitbone on the left side
shifts, drops or pops when pedaling. I'm not sure how to describe it
but it's like sitting on the edge of something and when I pedal it
falls off. Sometimes I can shift around and it will go away and
sometimes after riding long distance (over 20 miles) it seems to get a
little better. I've also have tried three or four bike shorts and just
regular shorts to make sure it wasn't the padding. Does anyone have
any ideas as to what it could be?

Thanks,

Paul