Hmmm with the T-Mobile sim of my BlackBerry the N900 not loose the signal perhaps my o2 sim is really broken ... but it works in the 6021 fine.

I had network problems after upgrade to PR1.3 too. It would lose connection in low signal areas and wouldn't try reconnecting in high signal areas.
The problem went away after I shutdown and removed the battery.
I upgraded ota, maybe the ota upgraded doesn't work so well.