Internal server error and Premature end of script headers

I am running buddypress and marketpress and am getting a LOT of errors. On the front end it shows as Internal Server Error. In the admin area, it's usually just a failure to finish doing what it's doing and/or a 404.

I read some threads here which said it might be the server just couldn't handle it (I am up to 150 sites on buddypress now). So I switched from shared hosting to a VPS. The VPS has massively more memory available to it than I am using, but I am still getting errors. Not as many admittedly, but still enough that it's unworkable.

I checked the permissions on my folders - all set to 755 as recommended.

The error log for every error is "Premature end of script headers:" which is supremely unhelpful.

The 500 errors will happen at any time, doing anything. Refreshing a page does it. On the back end it seems to mostly be caused by creating sites. Actually, I had a bunch of sites not create correctly when I was letting users self register. The site would be there, but the user would not be associated with it. So it's failing somewhere between the 2 things. is the website if that is at all helpful.

Would really love to find out what the problem is. Even having a recommendation on where to start looking woudl be great. Oh, this is hosted on Dreamhost too. I have found in trying to solve this that they are a bit notorious for this problem, but I don't want to move hosts unless I can be sure they are the issue.