Erratic 72W AC adapter

[matso]

When the computer is running on AC power, the AC adapter has begun to switch itself off an hour or so. Then, it can usually be restarted by disconnecting it from the mains for a short while, but eventually it won't stay on for more than a couple of minutes. Sometimes it is also difficult to get it going. I guess I have to buy a new AC adapter to my four-year-old A20p, but I am rather curious about what's going on. I would have expected the batteries to fail before the adapter. Does anybody know what is the matter with the adapter?

Thanks
Mats