Storm,
I refer you to my earlier suggestion. By removing one of your 2 pre-amps, this really hasn't proved anything. You need to go back to the basic setup - PC, RadarBox and standard antenna.
I would also suggest change of location now if you think that it might be external interference. Have you got a laptop, or a friend with a laptop. You can install RB on as many PC's as you like, as the software will only work when it's connected to the hardware receiver.
When I'm using the laptop with RB at work, moving the standard antenna on the windowsill as little as 5 cm can make a significant difference to my coverage and number of contacts.