Most STBs AV will drift if left tuned to a single transponder, causing a "lip sync" issue. This is because they only synchronize when the service is initially tuned. If you don't have a budget to purchase a commercial unit that continuously syncs the AV, an alternative is to program a consumer STB to go into standby for one minute every 12 to 24 hours. The standby/operate cycle will force the STB to reacquire the satellite signal and reestablish AV sync. I have successfully used a consumer STB with the channel timer set to go from the tuned channel to standby every 24 hours, 59 minutes.
An inexpensive alternative might be the GEOSATpro rack units.