Sounds like it's the phone, not the OS.
There are some members who have had their 3rd or even 5th replacement and still got problems but this might just be very bad luck (the number of users without problems is far bigger but it's the negative reports that stand out).
I got mine in November '13 and by today I only had one random reboot (not even sure it was caused by the OS, might have been an app, I really don't know what caused it) but other than this one time I've had no troubles at all.
You should try to get a replacement (tell them nothing worked - soft reset, hard resete, updating to newest software version - if they try to blame the problem on the software and if you still have problems after the replacement you still can give it back during the returning program.
Edit: additional: I've had far more trouble with my past Android devices (OS version not being updated - my first device is still on 1.5 -, dozens of apps force closing - even stock apps -, UI freezing up, unfixed problems - since the update to 4.1.2 my Nexus S' battery drainage is insane and no fix since it won't get any more updates,...). Considering this I'd say WP is a pretty stable OS compared to Android.