5 Groups of 3 = 15 tracks, that
could be pushing what the G4 will do.
Live was never optimised for Altivec/Velocity Engine (the G4's big selling point) so along with running Live 8, which is really a product of an Intel world, you _could_ be starting to push it.
Some additional strategies to try...
Render appropriate clips or samples into mono and/or 16bit for live performance.
(mono will halve the disk access, and using 16bit uses two thirds the CPU of 24bit, for that clip)
Disable all unused MIDI ports in Live's MIDI prefs.
Disable any unused inputs/outputs on your audio interface, at any possible MOTU level, as well as Live's Audio prefs.
Disable any effects routings (sends etc) that you know will not be using.
Render any Complex and Complex Pro warp settings to Beats/Tones where appropriate.
Swap any exotic 3rd party plugins/instruments you might be using for a native Ableton one if possible.
Also, what speed is your HD? It may be 5400 or god forbid 4200, which may not be helping.
Make sure you have lots of space free on your HD, like 10's of GBytes.
As a last resort, print more submixes...
hope this helps!