I am planning to pull out my old mz-r900 soon, and expect to have the same problem. When I first bought it (second hand), there was very minor oxidation on the terminal that pops out when you open the gumstick battery compartment. I scrubbed it all clean but no joy. When I plugged in to charge it would show up the "charging" message but would only work for seconds once I tried it on battery alone. I replaced the battery and the same thing happened. Eventually it seemed as if there just was not a good contact with the terminal, and this was confirmed because I could "squeeze" the top of the player (ie grip it at either end, where the terminals would be, and it would work normally until I relaxed the pressure. So I just found some copper wire and twisted it into a strand, and popped it in on top of the terminal when I closed the compartment. I used just enough wire so that there was a snug fit, and it worked fine. I think part of the problem was an old battery as well, but improving the terminal connections was a big help. This time around I might do the same thing, or maybe place just a little bit of solder onto the terminal.