I have the same problem since last year... See my thread http://talk.maemo.org/showthread.php?t=93854 - apparently noone was able to help me... So far, I live with it the same way as you - when I need network access, I switch to 2G only...

By thw way, I noticed that when I have something that constantly keeps the data running throught the connection (active open port and some data sent - could be minimal, but still), it keeps in 3.5G and is running... This is a silly and useless workaround (eats transfer fast), but works for some use cases - e.g. when I use software that has to do constant net i/o anyway it's ok, but for e.g. web browsing (which by nature open connections for relatively short amounts of time) it's useless, as I would have to disconnect and reconnect everytime I need to load next page.