iRule RAM CACHE odd behaviour

Go and look at all the profiles you have assigned and add up the "Maximum Cache sizes" from the config. That's the figure that you are using actually using per-TMM.

I assume you have seen this http://support.f5.com/kb/en-us/solutions/public/12000/200/sol12225.html?sr=11064657

Check your log for the error message - if you can't find it as you have lost the logs try updating the profile that didn't allocate (just change one thing) and save - you should see the error message then.

Go and look at what you are actually using compared to you Maximum cache size - you'll probably find a few you can reduce the Maximum Cache Size on.