When I use my N900 in the UMTS-Modus (3G) and connect the internet, I can see only for two or three seconds "3,5G" in the display. Then it changes automatically back to "3G". The internet-connection is still established, but I cannot use the internet with "3G". No datas are "running". Therefore I change always manually into the slower GSM-Modus for surfing with "2,5G".

I have this problem since some months. In the time before, anything was ok (same N900, same Vodafone SIM-card). I contaced Vodafone, as well. They answered, they checked it and anything should be ok. I shall check my hardware.

Yesterday, I used my N900 as a WLAN with "Qt Mobile Hotspot". In this case, my N900 established a "3,5G" connection and hold it. The N900 do not fall back to "3G".

Any idea, what I can do, to get a stable "3,5G" connection without a parallel using of "Qt Mobile Hotspot"?

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.

Dumb idea: do you have by any chance 'AutoDisconnect' (under system settings/connection settings) installed?
That is the only thing I can think of right now. (or possibly faulty HW? In case you would like to know install backupmenu, make a backup of root and opt to SD card, reflash and check)

Thanks for answering. "Autodisconnect" is not installed and has never been installed.

Short update: I put in a friend's SIM-card (it is not easy,to find somebody with a normal SIM-card - everybody uses micro cards). With this card, my N900 works fine. Possibly my N900 stopped liking my Vodafone card

Funny update: The test SIM-card was from "Telekom" and this one works fine. Now if put back my Vodafone SIM-card. Now: It works.

Oh really?
That does not make sense at all (but what makes sense in SW related quirks nowadaus? ).

I guess the full power cycle (needed to get access to the SIM) did the trick.
I had that once when the FM transmitter did not want to work. I tried everything (killing, restartong, rebooting, ...) but to no avail. Then I thought: hey, the only thing you did not try was to pull the battery. After that it worked flawlessly (up to now)!?

So misiak, try that
power down
remove battery
wait at least 10 (better 30) seconds
to be on the safe side, push power button for 3 seconds
insert battery
boot and try

Same here.

I've also done some tests like that and it works with cards from other networks, but doesn't work with cards from my network (Play, mobile network in Poland). Sometimes I get similar results as you - it works fine for some time (like 2-3 times out of 50) and then fails again... I've ruled out battery problems (can happen or not happen with either fully charged new battery or as well as almost dead battery with <10% of charge) and few other issues (I made a backup with backupmenu, reflashed -> same issue, then restored backup), so far I suspect maybe something my operator's infrastructure might be using (maybe different provider of "their-side" devices that don't talk well with N900's modem?), but this is a wild guess (however I need to check if maybe these problems are related also to my location - if I would be 100% sure that these 2-3/50 successful trials all happened near the same antenna tower, I would only have to also know if that tower is anyhow special - e.g. uses older equipment or something)... I have no idea however and, as I stated in my previous post, I switch to 2G whenever I have to use mobile internet reliably - I gave up on it and don't want to get angry on my N900 again

EDIT
I've also tried that, not changed anything in my case, but thank you for suggesting this.

Hmm, then I would say it is your operator (network related), as you said.
But calling help desk in such cases is as pointless as calling the speaking clock!

I agree on that, I tried calling them and the person on the other side of the phone knew less about that stuff then me, and instead of trying to help me with technical issues, tried to sell me a more expensive monthly subscription (like that would change anything...), so it is indeed pointless Reminds me of old xkcd strip - http://xkcd.com/806/