I really like Jammer. It helps people like me to jumpstart the creative process. I used to use it quite extensively.
As I understand it, the Jammer Pro plugin can only be played from Sonar. Apparently, this is due to the plugin specification which doesn't allow a plugin to tell a host to play, stop, etc.
I was able to get around this by configuring Jammer Pro as a MIDI controller (using Maple MIDI virtual MIDI cable) simultaneously with the Jammer Pro plugin. It was a long time ago, but as I remember, it worked quite well. It allowed me to use Jammer Pro to "build" MIDI tracks, playing the synths through Sonar as a VST host. When the Jammer Pro plugin was then inserted as a MIDI effect on a MIDI track, I could then control Jammer Pro from Sonar.
Like you, at the time, I was running Windows XP on a much less powerfull system then you have.
Here's a link
to how I set up Jammer Pro to use the TTS-1 in Sonar as a VST host.
Hopefully this will get you the functionality you are seeking,