I would suggest moving the receivers with issues to the location that does not have the problem. If the problem follows the receiver to the good outlet then you know your issue is more then likely something to do with the receiver. Same goes for the good receiver, take it to a bad location and see if it's still perfect.
However I doubt that is the issue, the receivers are usually fairly robust and can take a beating, I don't have DirecTV, but with DishNet I only had to replace one receiver and that was due the satellite ground getting cut and then a fairly close lightening strike.
More then likely I would believe the culprit would be a bad F connector either at the receiver itself, or on the switch. They can also corrode sometimes depending on the types of connectors used (kinda like a galvanic reaction), if that is a possibility (especially outside connectors) try disconnecting the wire using a fine wire brush to clean it, and then securing it back tightly.
Also looked for pinched wires. Coax is not meant to have tight radius's corners, treat it just like you would a computer network cable with at minimum a 1-2" radius in bends in the cable. Also stapled wires can cause issues (especially if the installer puts a staple through the wire). I've had this happen to me many times, and sometimes it'll work great for awhile then your SOL.
And finally there is the switch and the LNB's themselves. If none of the other things fix your issue, you might as well just place a service call to DirecTV and let them get a guy out there that has the right switch for your particular setup, and LNBs, and can make sure the dish is aligned optimally for you. Yes you could do possibly all this yourself, but the time, cost, and waiting for parts all added together really isn't worth it.