Hey everybody,
sorry for bumping into this old thread, although other forums discuss the same issue and are older than this... I thought I might give it a try here.
I'm having the exact same problem. To be precise if you search on google images for "nes wavy lines" you will find exactly the same. Also, the sound output with a/v cables is lower than when using the RF switch. I believe this is definitely not a problem with EM interference. I have tested my nes on many places replacing many accessories and it does this all the time. Many other forum threads are out there first recommending to shield the nes from EMI but that seems to not help. The other only idea that I see people talking about is that the big capacitor (2200 uF, 25V) must be the culprit. Unfortunately, I haven't found anyone who finished the process and reported the success or failure of replacing the capacitor. Does anyone know if this is indeed the reason for this common problem?? Also, I'm willing to do it but my soldering skills are minimal so any pointers would be appreciated. The last link posted by duke togo seems like a good start but it doesn't say exactly when this replacement is a god idea or if this has anything to do with this particular problem. Any help?