I'm posting mainly to see if anyone else has the same or similar problem. I searched the forum and bugzilla and the closest I found is this bug report: https://bugs.maemo.org/show_bug.cgi?id=10074 .

Recently some strange problems started occuring during connecting to mobile internet via my N900. What happens is:
1. I connect to the internet using 3g
2. Upon connection, the indicator of carrier signal strenght comes from 3g to 3.5g
3. The connection works for some time (it switches to 3g sometimes, but the upon clicking a link or performing some action requiring data download it switches to 3.5g for a moment)
4. Suddenly, the connection switches to 3g and stops working (internet browser doesn't work, ssh connection is closed if I am connected to any remote server, pings from terminal don't work). The N900 still shows the icon that it is connected to the network. The only way to make it working again is to disconnect manually and reconnect (and then the problem usually occurs almost immediately - after downloading one web page for example).

I thought it was maybe the weak battery at first (it was still the original one used all the time since I bought the device in 2009), but after buying new one (decision made after consulting the battery testing thread here on tmo, new battery is enough for 2-3 days of my typical usage) the problem persists. The device runs totally ok if connected on wifi, so I thought then that it is either software problem, modem hardware problem or sim card problem. I moved the sim card to another phone (N95 - it's a bit funny as the guy in bugzilla also used N95 later, but it's irrelevant, just a minor digression) and it worked ok for 2h of constant internet browsing.

I don't want to reflash right now... I'm on CSSU stable with this device and I can live with lack of internet access from it temporarily (I have my shitty old netbook with 3g modem with me most of the time anyway). And I doubt it is the software problem... The logs are pretty clear (I checked syslog mainly - does the modem have its own log or debug interface in the system or anything?)

By the way, as I mentioned, this device is over 4 years old (bought in 2009). I never reflashed this one (I went via all PR and all CSSU stable releases incrementally). The problem started appearing out of the blue this summer.

Can it be similar to the problem when N900 stops recognizing sim cards at all (the one that is caused by faulty simcard socket glueing to the board afaik)?

Any help greatly appreciated

As fas as i can tell its network problem nothing wrong in your device . I sometimes have the same kind of problem and yes when i try a diffrent sim all is fine

This is exactly how my phone behaves on my home WiFi in certain places in the house. The culprit is weak WiFi coverage in those places. The phone still thinks it is connected but somehow loses sync with the router and needs reconnecting as you describe.

BTW, it's not just the phone. Other devices in the house similarly lose the sync in the same places. My N900 is actually the least affected. The worst is my Dell laptop running Windows 8. When that happens to that piece of ****, more often than not it brings the router down with it and I have to restart it.

In other words, I second nokiabot's opinion.

It bothers me because:
- I live in a big city, so the infrastructure is pretty good and signal strenght is at 100% even at 3.5g.
- It happens ONLY on my N900 (and I have pretty few other various devices using the same operator's network, including 2 usb modems for computers, one netbook with builtin modem and one more mobile).
- It doesn't happen when I take the same sim card to different device.

I wil try a different sim card in this phone, I feel stupid for not thinking about this earlier... Nonetheless, what could I do if this specific configuration (this sim + my N900) doesn't work, but all other configurations (N900 + any other sim card or this sim card + any other device) work? Would changing sim card for a new one (but with the same number... I mean I get a duplicate of this one) make a difference?

I know about problems with WiFi and I also sometimes have them, but this is different - for me the problem looks like my N900 fails to switch from 3g to 3.5g at some point (I would like to know the cause...). I will let you guys know after trying with different sim card

Edit: and thanks for your input!