[Q] Stock rom instability - Galaxy S II Q&A, Help & Troubleshooting

Good morning everyone
Seems to be that my GSII has serious issues running his stock rom (continuous reboots, not-working 3g etc.). I tried several factory resets, but nothing worked 100% . CM fixes some of my problems, but the only full-working custom rom is MIUI, that seems to be stable and very neat. Despite that I still prefer the custom rom (yes I'm probably insane), or a more traditional one like CM and I really would like to understand what kind of problems can generate such a massive amount of errors and reboots.
Could be a hardware problem? MIUI works fine, I can't explain why I still got problems with other ROMs.
Sorry for my bad English, thanks in advance

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Without telling us the version of the stock rom you are using, we can never give you any answers.

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I'm currently using
2.6.35.7-I9100XWKDD-CL161513 [email protected] #2 Kernel
Gingerbread.xwkj3 Build
That's not the original stock ROM, just one I found (here on xda I guess...)
Is there some kind of hardware-check I can run by myself, just for exclude manifacturing defects?

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if you want to check to exclude hardware defects, u could go to the dialer and press *#0*# .. it will give u a list of tests which allows u to pinpoint any hardware defects ( if it exists) .... but if the problem is with the 3g, you could reflash your modem..

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Initial problems were strictly connected with 3g's use, after a few hours of use i've noticed slowdowns using sms, then it started crashing and random rebooting. That's the same reason why I initially switched from original ROM to CM. I mean, I know that Samsung's ROM is not the best one at all, but I can't figure why it's so instable on my device.

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fra beone said:
I'm currently using
2.6.35.7-I9100XWKDD-CL161513 [email protected] #2 Kernel
Gingerbread.xwkj3 Build
That's not the original stock ROM, just one I found (here on xda I guess...)
Is there some kind of hardware-check I can run by myself, just for exclude manifacturing defects?
Click to expand...
Click to collapse
As that the oldest stock rom try a newer one .
jje

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Updating: now my GSII works with latest stable Cyanogen, except for the 3g that's still not working. Should I reflash modem, as suggested by roophz?

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That old kernel is known to cause random reboots.
Update to a newer stock rom.
Sent from my GT-I9100 using Tapatalk

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Update 2: I switched from the old stock rom to the latest cyanogen stable version, then to a newer stock rom and then back to MIUI again. Still no life signals from 3g.
How is that possible?