Danny,
I'll through another one out to you! At my store I have tried certain midifiles in a PSR 8000, then the PSR 740, then the X1, then the WK8, and finally the EM2000, and they all play differently. My only explanation has come from AJ many months ago. If I had a Solton MS 60 and tried playing some of my midifiles from that machine, some of the drums do not line up because Solton was using user drum sets. I think that what you are experiencing is that the producer of the SMF you are playing was really not following the GM specifications. When I play a tune 1000 song in all these keyboards they play fine, but when I try certain SMF's I've downloaded from the net, this is when I experience problems.
This is only a thought, I don't know if I'm right.
George Kaye