I've had this problem at times, but it doesn't do it if I just do a quick restart every other day or so...like any computer, its good to reset every once in a while. Any device you get will be buggy at times...I have to say the Sensation 4G is far superior to the last 2 devices I've had, the blackberry bold and the vibrant. I will never go back to a Samsung after the vibrant...what a piece of junk.
I have had this happen to me. Sometimes when you loose signal you can put your phone into airplane mode and then bring it out of airplane mode and this might fix it with out a reset. You also might tried a new SIM card, sometimes people use their old SIM cards in newer devices and it can create anomalies. This also might be a problem with your location, if it only happens say at work and not at home.
Magnetar had some good advice, especially switching Airplane mode on and off. You can do the same with Network type (switch to GSM only and then switch back). This is mostly just helpful to avoid a reboot.
This may also be caused by apps you have installed, especially battery saver apps. Some of them disable data rather forcefully when battery gets low, and this could explain why it happens once a day. You should uninstall any battery managers or task managers that do not come on your device, and see if it helps.
Also, please try this: Wipe cache partition to clear system memory: HTC Amaze 4G / Sensation 4G / Wildfire S / T-Mobile G2 / myTouch 3G Slide / myTouch 4G / myTouch 4G Slide -- It is not a master reset, so you don't lose your data.
If your problems continue after you try all this, please let us know. Somebody should be able to help.
This has started happening to me more and more. Originally, it was only occasionally, but now it seems to be happening every week, or more.
I tried the Airplane mode trick, but that always seems to crash the phone, and force a re-boot anyway. Sometimes it crashes going into the mode. Other times, when trying to get back to normal operation.
But like the others here, I just want a phone that works like a phone. I don't want to have to check it every 5 minutes, to make sure it still has connectivity.
I just tried the cache wipe, so let's see if that helps or not.