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A little more detail please: do you mean it dropped an inch then rose an inch and is now stable? Or that it is continuing to drop, then rise, then drop, then rise?
If your overflow was somewhat blocked, then it makes sense that your water level would drop right after cleaning it since more water will now be able to flow through through your overflow down into the sump in the same amount of time. Then, once your return pump pumped the 'extra' back up into the tank and got caught up, it would be back to normal.
If your water level stayed an inch low, then you would need to increase your return pump a little (if it's adjustable) to compensate for the greater volume of water now flowing down your overflow.
If it's continuing to drop, rise, drop, rise, then I'm not sure what is going on. Maybe air is getting trapped in your overflow plumbing, then 'burping' out, then getting trapped again, then 'burping' out, and so on--causing your water level to bounce up and down.
No way to know for sure without more details.
It keeps falling and raising. I did hear some bubbles in sump from my drain but if it has air trapped wouldn’t it return to normal after bubbles ?
It would return to normal, until more bubble became trapped ... causing the cycle to repeat itself again, and again, and again.
Or it could be the opposite: a full siphon forming, causing water to drain quick, then air entering and breaking the siphon, then another full siphon forming, then air entering and breaking the siphon, etc.
The first thing I would try though is turning your return pump down just a bit. Your water level was stable before you cleaned the strainer. The clean strainer means water is passing down the overflow faster. So dialing back your return pump just a little so slow the water flow back to the rate it was at before your cleaned strainer--eliminating any excess air or siphon that is now being caused by the faster flow.
If that doesn't work, then:
Maybe try this airline trick if a sporadic siphon is forming:
Or this one if too much air is getting in:
Sorry, but without being able to see how the rest of your setup is plumped, and how the flow is behaving in your lines and your sump, I'm at a loss as to what might be going on.