I've tried leaving the highway and coming back multiple times. I've also tried restarting the console and loading my save again, but nothing works. The booster guy never appears, and my bike doesn't have a booster. Do you have a save near that part of the game.. Even a bit before then would be fine...

which firmware do you use?
try upgrading to the lastest custom firmware (3.80 m5)
if that doesn't help you have to use the 4GB stick.

There are various problems with homebrew and iso games from 8GB memorysticks...

I finished Full Throttle without any problems.
If someone doesn't appear try to leave the highway and get back on again.

The game is fully playable with the psp port.