Hmm.... I used to use a high-speed serial port with my USR Sportster 56k on Amiga.
On the normal port, you need to lock the baud rate to a lower value, otherwise the modem will outrun your port, hence causing gibberish.
I can tell you I used to use
AT &F
AT &H1 &B1 M3 S10=255
Couldn't tell you what those all mean, anymore, though. I just remember it was fast and stable. Not sure if the lock baud rate command is in that, or if it was a dip switch on the modem, though. Been several years since I've endured the torture of dial-up.
