I can see you are really fed up but it most likely IS your internet connection. I had the exact same problem myself. It started just after I moved home last year.
The disconnection seemed to ONLY happen when I was logged into SL. I could play any number of other games, browse as many web pages as I liked, but the only time I lost my connection was when I logged into SL, it almost drove me and my friends nuts! Over the last 9 months I had tried every viewer I could lay my hands on, spoke to every helper/dev/tech geek I could find. I changed ALL my cables connected to my PC, cleaned the inside of my PC tower, had my internet card/video card all checked. Unstalled and reinstalled drivers. Checked for virus's, spyware etc. Called up my ISP and moaned, cried, whined and even shouted once.
Eventually my ISP sent out an engineer to check my line and it WAS faulty. Showing noise on the line that wasn't picked up when the ISP checked from their end. They changed some equipment and in 20 minutes I was able to connect to SL with no problems at all. I don't have a particularly fast speed on my line but since that day I haven't crashed or lost connection one single time.
I know its not the answer you want, but its mostly likely IS your internet connection.....