Here is my GS4 64bit issue as best I am able to document:

This machine is windows xp x64 edition version 2003, with service pack 2

I am using a core duo 6300 @1.86GHz with 4 gigs ram (thought it was 8 when we talked earlier)

I have the last release of GS4 (4.01.0023.90421)
Hammerfall HDSP 9652 w/ driver version 3.06.3
MIDI Over LAN cp3 (3.2.6 build 221)
several sata hard disks (only 7200 rpms unfortunately)
on my GS machines I mostly use VSL full pro edition, which I am registered owner of, though many other instruments/samples as well
no other software on this computer, dedicated and standalone

I have a template I created that uses 74% of RAM (I made one much higher, just a hair under redline, but after I started having this problem I trimmed it back to see if that was the problem)
I have many self created stacked channels with keyswitch (mostly my lovely VSL stuff stacked as it should be ;-)
I set up 23 stereo inputs and named them (like violin solo, violin section, viola section, etc etc) so that I could use different panning and amounts of gigapulse *in gigastudio* to keep the load light on my mac while i am writing, and then another setup dry and centered when its mix time
I then take all my loaded channels and ports and assign each sound loaded to the various named inputs
For instance:

Port 1 Channel 9 is Westgate flutes stacked, on the far right the input is selected to "Flutes" which corresponds to input 41,42 in the DSP station, so I can pan and occasionally add effect.
This is then routed to Group 11, 12 to share convolutions with other similar woodwinds, and then outputs all of these grouped woodwinds to output 17,18 of my HDSP (and I send all gigapulse back up from all instruments to 23,24 so I can isolate dry sounds in my DAW)

Now seemingly randomly, every so often the inner demon of GS comes to life and it will reset ALL my inputs selected in the MIDI select menu to the default (so port 1 channel 1, port 2 channel 1, port 3 channel 1 etc etc [all ports channel 1] gets set to input 1,2; port 1 channel 2, port 2 channel 2, port 3 channel 2, etc [all ports channel 2] gets set to input 3,4 etc on down the line.

Freaky? yes. Annoying? yes, especially in the middle of a session where every 20 minutes is invaluable or priceless when its a moment of creative inspiration. End of the world? nah, just annoying. But the end of the world comes next

So I start at port 8 channel 16 and re-assign back all my channels to the correct inputs, and somewhere along the line, in different places, usually when I am re-assigning a large stacked channel (like a channel with all violin crescendos and decrescendos and other dynamics like fortepiano etc) and then GS4 freezes for a second and I get blue screen of death, forcing me to reboot my machine. These days I dont even bother to re-assign the channels, I just keep working till I can take a break and deal with weird sounding effects on wrong channels and then I just reboot, much less annoying.

So this makes my investment in GS4 and in 64 bit hardware and OS pretty much useless. While I have been thrilled with the new GS4 features, I feel once again betrayed by TEAC for not supporting their developers to have proper testing and support of this product. Another reason why I was simply ecstatic to hear about the Garritan purchase of Giga!

Anyhow thats the whole thing as I mentioned earlier on the chat, I hope this information is useful