When troubleshooting any situation with a bad/spotty picture quality, I think removing as many variables from the situation as possible is the best first step. If you have something between your aerial and the tuner, the best first step is to probably remove anything superfluous and try a direct connection.
Once you have removed all unnecessary steps/parts in the line (including additional network hardware, such as additional switches or wireless APs), then slowly add back in one device/step back into your setup and test again. Wash, rinse, repeat, until you find which added step caused the problem.