Xircom REM56G 10/100 slow modem

Have troubles setting XIRCOM REM56G 10/100 PCMCIA modem/ethernet card on IBM 2635 2ao, W98SE.
Took me a week to make LAN work. Now LAN is OK, but modem is OK only donloading (full 5.2+ KBytes/sec constantly).
Upload starts OK, then slows down after abt 2 : 5 seconds, and stops.
It looks like simultaneous up/downloads worsen the situation.
IMPORTANT: During installation, I had a situation where simultaneous down/up to different ftp sites gave constant 5K+ (almost) without retries.
Have latest firmware upgrade an latest drivers available on Intel site. IR is disabled on local profile.
Thank you. MIke.