I have just recently started using more of the RXP plugins now and its strange, if I insert the first one and import a REX file into it and play it along its fine.
But then if I insert a 2nd RXP into the project and play another REX file within it, both the synths cause a high cracking noise and the metres on both these tracks will randomly fly past the highest threshold db mark and continue like this untill I remove the 2nd RXP insert synth then all is ok again.
 
I can't find a thread on here similar to this. Has anyone any ideas.

Computer specs? Soundcard? Crackling usually means your computer or soundcard can't keep up. 

Not sure, I am suspecting issues with my soundcard for which I have contacted NI, I do use my Traktor Kontroll S4 as one (soon to be replaced by a FA-101 or MOTU mk3 Ultralite), but I can run lots of other synths and plugins with no issues, only when I load 2 RXPs together.

Strange!  

Not getting the new interface until October, so may see then.