Cache Plugins with Other WPMU Plugins

Hi, I have a site that uses your Membership plugin, amongst others. I was using the QuickCache, which is no longer being supported. I have often had problems with a password protected page (that uses a membership levels) for access to a page with downloads (using the Download Manager Pro plugin). The error ‘Ajax failed’ has occurred quite a few times. Clearing the cache fixed it.

My question is which Cache plugin should I switch to, and how should the settings be configured?