Hi, Blane:
This is something I've not run into before with Notation Software products (and my involvement goes back before it was even called Notation Software!), so I have to think it's something specific with your computer.
Your suspicion is that it's a memory problem. Neither Composer or the MS GS synth use that much memory, so I would question whether that's likely, unless there is lots of other stuff running at the same time.
I suggest you install something like Coolsoft's VirtualMIDISynth (
http://coolsoft.altervista.org/en/virtualmidisynth). It's free and you can test the problem with it. Be sure you download one of the suggested GM soundfonts to go with it. If you like it, you may even stick with it.
As with all software synths (it's actually a sample playback device, not a synthesizer), it has some latency, but apparently less than many. I'm not sure if it's ASIO4ALL compatible or not.
David