Casio keys not working properly

[Langgil]

Hi! I have a WK3500 with the following problem. If you start from the extreme left, count the keys ( including the black ones) and starting with keys # 8 and then the keys # 9-10=11=12=13=14=15 keys don't behave properly. Even though you touch one of those keys very slowly or very light, the sound will be full loud. If the patch you chose is let say a "piano" patch, not only is the sound full loud but it's also like the sustain pedal is on. The sound slowly fade and after that... that key is "dead" . You can't use it anymore unless you switch off the keyboard and turn it on again. If the patch chosen is let say "Strings" then, the sound is full loud and "jammed" on sustain ( full loud jammed on sustain, no fading out). The only way to stop it is...to turn off the keyboard. The only way I can use this keyboard is to use it connected to my Roland Midi controller. When connected to the Roland, all the keys ( on the Roland) work like they should. I know the problem is because of humidity but...the Roland work just fine. I also have a small Yamaha and 3 sound modules that all work right in the same humid environment. 2 days ago,I did place the Casio in front of a propane fan heater and after 45 min. it would be working properly. The following day, it was again not working. I did place it again in front of that heater but this time, after 1 hr. , it still was not working right. "Which part" is affected by humidity ? Why only 7 keys are not behaving right? I did open it and did clean the key's contacts 3 different ways but to no avail. Thanks!