Im wondering if you have a channel on it or not. I had trouble w/ a mod that I was using for a similar reason. I swapped it out w/ a second to find the same trouble. I found that connecting it to the antenna aray on its input stabalized the mod. The conclusion I drew was that the mod had a agc and it was trying to make up for a fluctuating input that was not there. Now this was a long time ago in the stone age of cable head ends but it may still be so. Hope it helps.
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