I was having this nasty problem where my ajax toolkit Tabs in a custom module were having their visibility set to hidden.

 After loading up Ajax Control Toolkit into VS2010 and breaking on the part that controls the rendering, I noticed that the Render method is only called sporadically.  So I looked through the module settings for a way to change that. Turns out under:

 Host -> Extensions -> [Edit this extension] -> Module Definitions

There's a setting called "Default Cache Time" which was set to 60 by default.

Setting this value to -1 (never cache) forced the Ajax Control Toolkit Render routine to be called (correctly) every page-load.

 Removing and re-adding the module to the page fixed everything!

There may be some Partial Rendering setting somewhere that accomplishes the same thing, and I'm not sure if this was the optimal solution. Any input on the "best" way to handle this would be appreciated.

 TL;DR: Telling the module to never cache causes Ajax Control Toolkit rendering methods to be called every pageload.