Would be interested to know if you can tune the signal to improve the SNR, but ........ I guess it is also possible that you are getting too good a signal on 28.2E and swamping the tuner, depends on LNB and cable run.
Would explain why the others are OK (7E etc)
The only way round would be to stick an attentuator on the cable and play, but obviously this is fine on a fixed dish but would mess up your other sats on a motorised.
Life can be real fun with sat dishes I started with a 1 metre dish on the roof and now look what I have stuck up on the garage roof bolted down and with heavy concrete slabs as extra weight