seq not playing in time

I'm having trouble with a sequenced bit of music playing from a seq object.

It's playing a 16 minute long monophonic track to 2 channels on an external hardware synth and 8 instruments in Reason.

In Reason, I have 4 each of the 2 kinds of sampler instruments. Each group of 4 goes to a 6 channel rack mixer and I have a key on my computer keyboard to turn the volume on and off (press down = on, up = off) for each of the 2 mixers. I use this alot in 2 sections of the music, not at all anywhere else.

In these sections where I am doing this though, it seems to throw off the seq object causing it to get a little slower and back up to speed again... it's like playing with someone who has really poor time and there's nothing I can do about it.

Now, recently, it's also been doing the same thing in the whole rest of the piece too. I'm not sure what could be throwing it off, I don't think there's too much MIDI calculations going on, and even if there were a lot, I didn't think it'd have this effect. Anyway, I'll post this part of the patch that's giving me trouble when I get home tomorrow.... meanwhile, any ideas?