Ok I've found the culprit - I had triple buffering set - speed stayed at 99%. Now 100% with no sound jumping. This is strange as I've used this setting as my default to prevent screen tearing (e.g. in Bank Panic).
Could an upgrade to Win 8 be the reason as I have to use the default windows graphics driver for my ATI Radeon 4600 (Catalyst no longer supports my card)???
EDIT: Setting triple buffer for Bank Panic makes it run at 98% with sound jumps. Any ideas? My processor runs at 2.6 Ghz