I have a 400W inverter made by Coleman, I had a similar problem with mine, only it wouldnt work at all (I think, I cant remember if I only tried using my playstation 2 on it or not). Anyways, I called the tech support number and they had me do a couple of tests on it, and they said to send it in and they would send me a new one. (They also wanted a copy of the reciept, but I bought it off ebay so I didnt have one, and they still gave me a new one. :wink: )
Hope this helps