sudden CPU spikes with VST's


I am using Slate Digital and T-Racks VST’s in Wavelab. They were working perfectly up to about 2 months ago, when I started to experience CPU spikes on playing a track with those VST’s enabled. The same is happening when I use them in Cubase 6.

Up to now I have been able to use both those VST’s plus a De-Esser when mastering a track, now when a track plays back it is ok for a short while, but as soon as louder music kicks in, the playback stutters, then goes silent, even though the track bar is moving.

I can successfully render a track in Wavelab using the VST’s, but the tracks won’t playback.

I have not changed my Latency levels in my RME soundcard, and changing the levels makes no difference to this stuttering problem.

I can open Waves VST’s and Steinberg legacy VSTs and use 2 or 3 of those with no problem.

I have tried uninstalling and reinstalling Slate Digital and T-Racks, but no difference.

Any suggestions?