Another possibility is a drifting LNB or LNBF. Could be temperature sensitive, but more likely would be the LO drifting when the LNB/LNBF is directly in the sun. Take a small plastic garbage can and slide over the device, and see if the problem goes away. If this is the cause, it is either LNB replacement or in the case of super sensitive and weaker signals, cover everything. If you have a dark colored weather cover, paint it flat white with a spray can. One can often duplicate thermal drifting problems by getting a hair dryer close, but not too close, to the electronics and see if things drift. Before spending any money on filters or anything else, check out the above basics---old electronics is often not stable enough to deal with DVB-S2 digital signals. Good luck.