I'm having the same problem. Usually, new telephone or cellular
installations causes this. Signal quality goes up and down like crazy, there are periods of good quality, to again have jumping signals. Rainy days are sometimes TI killers, so you'll better reception in those days.
Look for any new RF tower, with all the new WiFi and cellular 3Gs and so on, TI is more prevalent than before.
Countemeasures:
1.- Complain with FCC authorities, they are obliged to send a technical team to measure the source of the interference
2.- Reposition your dish behind a building or fence so you isolate the interference
3.- Shield the dishes with a fine metalic mesh frame or frames acting as a wall (mosquito mesh is fine). Cumbersome eyesore, but sometimes efective and inexpensive.
4.- Anarquic actions (not recommended, but sometimes is the last resort)
Google "Satellite Footprints by Dish Size - Noise Discussion, Terrestrial Interference ", it's the best down to earth explanation I have found.
hope this helps...
Saludos