We may have the same problems, sir, though we may have a very slight difference on the device we're using. I haven't tried using Galaxy S so I can't really vouch on this to work on your device.
Recently, I have experienced that too on my Galaxy S II. Here's what I did:
1) I cleared my browser's cache from the application...
2) I formatted my SD card using the SD Formatter from this link...
3) I installed WatchDog just in case something goes wrong.
4) I move all my apps to my phone. No more apps on my SD.
It was quite hard to narrow down which one was the culprit because I did all the abovementioned procedures at the same time without testing which of them works or not. But ever since I applied those procedures, I never got any random reboot up to this point yet.
User Theonew also mentioned that it might be #4 that caused it. If it won't take much of your time, you can give it a try. But I suggest you apply #4 before trying the rest.
Another thread also mentioned that Dolphin HD browser was the culprit on their device. I cannot comment on this because I was having random reboots when, in fact, I don't have Dolphin HD on my device.