Hi, Graenoss -
I do have a Casio WK-1200 (in addition to the Casio WK-3500. This may or may not be your problem.
If you do have sound for awhile, but it just seems to lock up, then (and this was documented on the web at one time) make sure you have fresh batteries in the keyboard. Even if you are using the power supply, there was a problem with the 1200 in that it would lock up on you at times. I have found that when using the power supply, the batteries do last a long time, but they do need to be changed every so often. I usually experienced this when the sound started dropping out.
This may or may not fix your problem, but it's worth a try.