Keep getting 500 Internal Server Errors

I am trying to add a few plugins but I keep crashing my sites. I have tried different domains, different sites, different setups but I keep getting 500 Internal Server Error crashes. I have been trying to discover which plugin causes the problem and it seems to revolve around Pro Sites. Now I can load Pro Sites by it self with out a problem but in this last crash I added Market Press and my site crashed. So on the site I have the WPMU Dashboard, Pro Sites and Market Press, when I activate Market Press I get the 500 error. To recover my site I access the directory and delete the last activated plugins.

I am not sure it is only this combination of Pro Sites and Market Press that causes problems. In the past I have tried adding Membership to a site that has Pro Sites and experienced the same error crash.
Does anyone have any thoughts?