3.0b performs better than 4.0c on my SD1

It looks like 4.0c works well for some users but it does not work for me. It behaves the same way 4.0b version does - eating 2nd voices after fill both with new and old sounds. It is true I only tested it with few styles using lots of strings and pads, but in 3.0b they play without problem.
I installed 4.0c directly over 3.0b. Is it OK this way or do I have to go through 4.0a/b versions?
Maybe I have some other problem in HW.
It seems I will stay with 3.0b though I miss some new functions.
Otherwise I have great time with SD1!
Luka

I've just read again posts about improved response of 4.0c polyphony…… Could it be that I introduce those dropouts by calling styles with single registrations (which were made in previous releases) and pull in something that doesn't fit into new release? I have to check that when I come home tonight.
Could somebody with good experience with 4.0c tell me how he tests polyphony and what is better now?
Thanks
Luka