This is the same problem that was discussed earlier in topic 59. Using the latest rev 3.1.19 i am still getting intermittant hangs with the serial ppp links. Sometimes the transfer of a large file will hang at 5-7 K downloaded for for 3-5 minutes and then continue on to completion. on older versions of the pcmcia tools(~3.1.16) the downloads completely stalled after about ~70K download.
looking at the discussion forums at http://pcmcia.sourceforge.org/cgi-bin/HyperNews/get/pcmcia/serial.html there are duscissions of interupt conflicts and ioport problems. Thread 12 covers this in some detail.
I am willing to capture logs and recompile changes if someone has any suggestions.