Streaming through Roku causes DSL signal loss

[Calgarian_wife]

When I stream through Roku, a sudden loss of DSL signal occurs, usually when I try to switch TV channels. Restarting the modem does not help. After that the signal comes and goes within an hour and then stabilizes. Replacing the modem did not help. My Internet provider checked the line and says there should not be any problem. What might cause the signal loss?