If you phone is rooted, try to disable not necessary services and apps always in background, additionally install something like Advanced Task Killer and run it before using Street Pilot so you have clean Ram in phone.
I think also installing SP in sdcard is important to have it working fine. Crashing in recalculating route or exiting SP app during navigation is a Ram problem. I remember it was happening in GMXT on Symbian in my old N95 too, before using it I shutdown phone and restarted it to be sure to avoid problems in navigation.