[vdr] reception issues every 21 seconds due to EIT scanning?

When background EIT scanning is enabled, I am getting signal disruption about every 21 seconds, with my dual sony playtv tuners (2 playtv boxes each with two tuners).

The signal level is mostly constant at 70% with solid problemfree reception otherwise.

I've heard that the nova-t 500 cards can have issues with disruption when the second tuner on a card is tuning, so I presume this is a similar problem, as the hardware is mostly the same. Is VDR really tuning to a new channel every 21 seconds?

Is it possible to limit EIT scans to a designated frontend?