I reviewed reception comparisons at
this site, and realized that the 4228 probably has 4 or so dB gain for ch. 9, so adding a VHF antenna with only a couple dB more gain will likely not fix your problem. If you haven't tested with the preamp completely removed from the line, then do so first. I third the idea of local interference problems as a next check.
Several of your stations come from the same tower, including WJLA, WPXW, and WHUT. Do any of those have signal strengths that jump around (a classic symptom of multipath problems)? You might find changing the height of the antenna will help in that situation.