Re the interferrence, I find it unlikely that it has anything to do with LNB signal getting into the RF output. I'd sooner beleive that the coax used for the ch3/4 output to the TV either has a bad connection to the shield or perhaps has a broken or short center conductor. You might test the coax with an ohm-meter. If the rf cable is defective, or if you are going through any kind of "t" or switch, and if you have a local channel in that area ch-2,3,4or 5, you could get interferrence in there. You might also try changing the
modulator setting from ch3 to ch4, or visa-versa to see if that helps.
Re the channel changing slow, slow is a relative term, so you might describe what you mean more, in seconds perhaps.
Does it change slow when pushing buttons on the front panel too? Ie it might be a remote problem. There have been a lot of complaints re the
remote control on these things. I've found that my lifetime is VERY sensitive to IR interferrence, and it will behave strangely if there are computers or other IR devices around, and/or a lot of sunlight coming in the window, etc. You might try shading the receiver to shield it from other light, and then try using the remote. Also, the fortec remote control has a tendency to have the buttons stick. Make sure that none of the buttons are sticking on, or you will get strange behavior.