In all probability it is not that simple. You are as likely to roll back to CS4 and have exactly the same issue. You would be better trying to fix your problems with CS5 with help from this forum, than rolling back to CS4 in the hope it will work out for you. It's probably moot anyway, as the only way you'll get a copy of CS4 is to find someone with an old disused license, who is willing to transfer it to you.
So to get the ball rolling, what exactly is happening with your Panoramas and Focus Stacking?
CS5 was known to break causing scripted functions to not work properly, for instance. You can test this by:
Loading the files into Photoshop Layers from Bridge.
Select all the layers, and go Edit > Auto-Align Layers
followed by Edit > Auto-Blend Layers with Seamless tones and colors checked. If that works OK (you can use it for both Panoramas and Focus Stacking) then you know that scripting is broken. That is fixed by reinstalling Photoshop.
I can do both actions that is not the issue. the issue is it performs these tasks awfully.
panoramma will blend layers that were perfectly clear and leave them hazy, grey, blurry and desaturated for no reason. usualyl these areas are not even near seams. it never aligns anything ever...
Focus stack just... jsut doesn;t work, the end result is typically it using the most out of focus areas.. the exact opposite of what it should do...