What were you doing that would have made the nitrates go down from 60 to 40 in the first place? Are you running macroalgae in a refugium or did you do a water change? If you aren't doing something specifically to remove nitrates they should just continue to just go up, not up and down and up again.
If you see nitrates go down and you aren't downing something to remove them, do the test over again. Testing error is a common issue. Try very hard to do every test exactly the same way each time and clean all the equipment after every test. Our club did a test and found that different people, testing the same water and using the same test kits got quite different results. But the same people doing the same test a second time got the same result they got the first time. It's just minor differences in the procedure of how the individual does the test.