Write two rules.
I if temp > x on
Another if temp < y off
Write two rules.
I if temp > x on
Another if temp < y off
The Rule 5.0 docs are probably a better source for this at the moment (the 5.1 docs are new and still missing some things, this being one from what I remember). But my suggestion is the one @672southmain suggested above, the "Repeat While (Expression)" feature.
Just here to say I just took this path to convert one of my Simple Automation Rules to RM, was going to go rewrite the rule in RM from scratch but decided to give this a try.
Using the migration path was a piece of cake and I can see how it would have the added benefit to help folks bootstrap their way into RM use.
Adding what I couldn't do with the rule in SAR was all I had to focus on thanks to all the rest of the stuff being laid out exactly the way it should be/needed to be. Seeing the basic construct of operations was in fact instructive.
Thanks.