3G will frequently display 'no service', then 'sim locked'... then crashes

I have an iphone 3GS, running ios 5.0.1, it is unlocked. (I'm in Spain)

I had no problems with the phone for two years, until a few months ago, when the phone began to randomly disconnect from the SIM card. It will say 'no service' in the menu bar, and instead of searching / re-connecting to a network, will then say 'SIM locked', and ask for my PIN number.

At this point, the phone will sometimes unlock and reconnect, but often it won't unlock until I do a soft restore.

This cycle can happen several times a day and I can barely use my phone.

When the phone is 'really' stuck, it will display a warning screen, saying that the iphone cannot send or recieve calls and I need to do a hard restore.

If I turn of 3G the phone lasts longer between 'crashes', but the crashes still occur.

I did a hard restore - last week - and initally this seemed to fix the problem. (Curiously, when this problem began, I noticed that I could no longer put my phone into silence mode using the side button. I thought the button was broken. However, after the hard restore I had no issues with the button, making me think the problem was related somehow...?)

Anyway, after the hard restore the phone worked perfectly for 3 days. Then I noticed my push notifications weren't working so I used SAMPrefs to to fix it... (not sure exactly what lingo to used - it has to do with reactivating the phone somehow?, I just followed the instructions...). Then the crashes started again. I should note that in 2010 when I first jailbroke my phone I had to use SAM to fix my push notifications, and it worked perfectly and didn't cause any issues in the phone.

I've read other threads about phones that randombly crash, but the phone never says "no SIM installed", just that the sim is locked. I've tried my husband's SIM from a different carrier and the phone acts the same.

So, I'm curious if the community has any suggestions. If people think its hardware or software, or if anyone has had a similar experience? Etc. I'm a poor grad student and can't update my iphone, I want this one to work!!!