It is near to impossible to record into one session and paste pieces into a new one. Even a small discrepancy could mess off your timing.
It is better to record one large session with multiple tracks. Even on my G4 iBook I have done up to 20 tracks without difficulty. Some pointers:
1. If timing is exact through the song use a drum loop as a click track. You can use a metronome but a drum loop helps you 'get in the groove.' When all track are done you can delete or mute the click track.
2. Record dry (no effects) so your timing can be exact. Delays and reverb can throw off your timing as well as be a strain on your processor if They are activated for many tracks. Some effects are heavy processor hitters
3. Watch your playhead (the moving triangle) as you record. If it is always turning orange or red you are pushing your processor. It should remain white.
MacBooks and iBooks have special energy saver settings that drastically effect Garageband performance. Make sure your energy saver is not set to the 'best battery life' setting. You need the high performance setting to get the best from Garageband which is a processor hogging app.
Other ideas to maximize performance:
http://docs.info.apple.com/article.html?path=GarageBand/4.0/en/6567.html
Kurt