I think what you are seeing is the "normal" variation in the
output of the MOSFETS. Some are better than others at RF.
Gordon's suggestion to turn back the drive is probably the right answer.
Alternatively, you could turn down the gate voltage a bit, but that
would probably introduce compression distortion. Or you could increase
the size of the heat-sink and see how it goes. Probably the wisest thing
to do is both turn back the drive a bit and increase the heat-sink.
Some, but not all, IRF510's are capable of 20W or more on 80m.
you might happen to have some good ones. The key lies in the
harmonic generation and the heat-sinking.
Note that what you are seeing MAY be from UHF oscillation included in
the signal. Be very aware of that. Check your output very carefully.