hi....yes, PLP 120 does not save your tempo converted files. It has to recalculate them every time you reopen the song. Don't know if that is causing your computer to crash....I did encounter a similar problem where converting an entire loaded bank on my g5 dual gig (Logic 6.3.3) caused the plug to become unstable/unusable.
Thanks. Michael Kleps from ReFX, the developer of the synth engine, explained the situation to me over at KVR. It seems ProSoniq, the creator of the Mpex algorihm, is afraid people might figure out a way to load their own samples, so they require that the PLP recalculates the data every time.