I actually had a similar problem when I was working on tts.. it appeared that ttsd slowed everything down so much that the Empeg became unusable. Then I tried again, this time I actually got in the car and drove around and the tts stuff worked without any apparent ill effects.
A couple things to try:
1. If you disable the tts stuff temporairly by making "tts" not executable (chmod -x tts) does Roadmap start working again?
2. Are you hooking up to an actual NMEA GPS or a simulator? I'm wondering if the position updates are coming so fast from a simulator (or fast updating GPS) that it's getting bogged down.
3. Are you testing in AC or DC mode? I don't know why that would matter as long as everything is getting started but it may be affecting the serial port somehow.