yet another GPS unit question

I have successfully connected an OEM GPS
unit to the PSP (http://www.lextronic.fr/laipac/tf30.htm) and
I can read/parse proper NMEA sentences at 4800
bauds on the PSP using my own software.
However when running Map This, neither
satellite counts nor position are displayed (both
show 0). Is there some kind of debug mode to
Map This in which I could just check whether
the serial port is working properly ? I have set
the baudrate to 4800 bauds in the config file,
set the slow_gps flag and tried a few delay
values ranging from 1 to 3 without success.
What other parameter can I play with ?