Update on the poor performance in Win7
I downloaded a sequencer (Reaper as I don't have the cash for the others!)
I have imported the same scores with which I was having playback issues in sib in Win7 with the same number of Altiverbs FX etc and Reaper has NO problem playing back..... zero glitches
I still think something's amiss with the Sib/VST/Altiverb combo in Win7.... at least on my PC!