Bad sound break up

Hi all,

I have been having problems with really bad sound break up and, after doing a little investigative work, have discovered something that could be an issue!
Firstly, you can see my set up from my sig, and I think it is man enough for my usage! I tend to record a lot of audio tracks (guitars and overdubs, vocals and overdubs and backing vocals etc) then overlay VST instruments.
Until recently, when I was using group or FX tracks, I used to route the Individual tracks to the required group via the routing tab in the mix console.
I recently started to use the send tab in the editor as it seemed to give me a better control of the sound, and that’s where the trouble started.
I began having really bad audio drop outs, very very loud crackling on all channels, the sound would speed up when being played back, and the cpu meter would be constantly in the red!
I closed all non required programes, adjusted power settings, updated everything but it didn’t seem to make a difference.
I did all the necessary adjustments in options page for Halion Sonic, and that made a slight difference but the issue was still there intermittently.
I was freezing every track I could but that didn’t make much difference. I was changing the setting for CI1 in the device menu which didn’t really change anything, then I was mixing a part of a song and noticed that if I turned off a send, all the sound problems seem to stop!!
I have just changed all the setting on a song I am working on ( 20 tracks, 16 mono audio and 4 Halion Sonic stereo ) by turning off all the sends and placing them via routing and it seems to cured all the issues but what I don’t understand is why?
The CI1 has a stereo input and stereo output so maybe it’s not big enough for my requirements although if that is the case shouldn’t there be a warning somewhere? I don’t set up mono inputs so is that the issue because all my vocal and guitar tracks are mono, is that a reason for the problems?
Could it simply be that I am missing something obvious regarding the I/O settings?
Has anyone else noticed this?
Can anyone from Steinberg give me answer?
Do I have to use the routing instead of the sends facility, when that is what the sends option is designed for?
Any help would be greatly appreciated!!

Jim B