Sounds like the locknuts must be moving relative to the frame. This could ne caused by
a) the nuts not being done up tight enough - how long a lever are you using?
b) wear on the frame dropouts.
c) the locknuts on the axle usually have knurled bits to grip the frame but they can get smoothed off.
d) your transmission is totally worn out.
e)You are just hellishly powerful and should enter the world track champs in Poland in two weeks.
a) the nuts not being done up tight enough - how long a lever are you using?
b) wear on the frame dropouts.
c) the locknuts on the axle usually have knurled bits to grip the frame but they can get smoothed off.
d) your transmission is totally worn out.
e)You are just hellishly powerful and should enter the world track champs in Poland in two weeks.
hope that helps.