you only need to change the default transponder when the box is first powered up - like after a power cut. when the box powers up it tries to download certain information from the sateliite it needs to boot up - time and 6 hours of EPG data. If it cannot find this data then it will show bno siganl found. Thats when you put in your other default transponder, which is a lot stonger, and allows the box to complete the download. Once that download has finished, nine times out of ten the defatult transponder will go back to 11778.
Changing the default transponder DOES NOT make channels stay on longer.