You were all exactly right.  100%.
I had the bouncing/erratic/no-control/jumpy/falling Spark issue off-and-on for a year!  Even with a second Spark.  It happened just occasionally, with no pattern or common flying conditions.  Drove me crazy!  I blamed the heat, I blamed the firmware, I blamed the downward sensors, the batteries, the motors...all the while, pilots in this forum (and elsewhere) insisting it was the props.  Either non-DJI or damaged props.  And I was flying with cheaper 3rd-party props that I had stocked up on.

But I refused to believe it.  How can the condition or quality of the props make the motors rev up and down wildly, and cause the aircraft to be unresponsive?  Didn't make sense to me.

So after ruling everything else out, and getting very frustrated, and being tired of the uncertainty I felt each flight, I finally said, OK....I'll pay full price and get some proper props.  Can't hurt.

That was in the spring of this year.   Haven't had a 'BOUNCY' incident since.  Not even one.  I make sure the props are seated properly every flight, and unfold/open every set separately, making sure they're all straight.  I never just let the motors "open" the props by centrifugal force, and I take off slowly and deliberately, not just jumping up from the ground at full power.  Since replacing the cheapo props with better quality brand-name ones, I've had an uneventful flight every time, and I fly a couple of times a week.

I'm a believer now.  To the people who advised me that the bouncing was a prop issue, I apologize for doubting you.  You were right.

Good for you, but did you try to do this:
"and I take off slowly and deliberately"
also BEFORE you changed the props?

Because that is a known change in the starting behavior, "NO app start,but CSC and slowly" , to get around the bumping.
At least for some time it was at mine.
And after that it was CSC in Sport Mode, gaining height as quick as possible , to get it away from the ground and let it stabilize itself in the air.

Since I am just flying DJI-props, also changing 'em every now and then and did that for testing during my one bird bumped up and down,  I am not so sure about "just the props"

But yes, I found out, that a little tiny cut at the tip of one prop can create some bumping behavior, too.
Only that was midair, meaning up in the air, and affected the steadiness at hovering, so that Spheres didn't work out very well, sometimes.

And I am almost certain, that I switched the blades from one to the other Spark, to give that "props are the troublemaker" a try.
Just in all the time, I had only the one bird bumping.

If it worked out for you, good for you.
But I  still have doubts. ;-)

Aside that I think, that DJI would have found and pointed that out, as number one thing to do.
Not the "calibrate" IMU hint.

But nevertheless: if it works for you, good for you
Perhaps some of the after market props are not as good as others and less good as ori props?And worth a test is it anyway.

I'd never use 3rd party props,  just not worth the risk.

S-e-ven Posted at 10-10 22:19
Good for you, but did you try to do this:
"and I take off slowly and deliberately"
also BEFORE you changed the props?

Yes -- I tried every trick/tip/hint I thought was plausible, and still the bumping start would happen occasionally.

I'm not saying new, name-brand props are the catch-all cure for everyone who has the problem, but for whatever my particular bugaboo is, this worked and the problem has not returned once.

But I do calibrate the IMU and compass fairly regularly, and I fly on the conservative & careful side.  I offered my experience in case someone, like me, was hesitant to approach the props as a cause of the 'bumping'.  And also to, in all sincerity, show my appreciation for the pilots whose advice I brushed off all this time.

We're glad to know that your issue was sorted out. Again, we strongly recommend to use Original DJI Propellers and also to replace the propellers once they are damaged, scratched, bent or being used for more than 1500 take-offs for safety purposes and to ensure flight stability.

I never used anything but the DJI props. Thanks for the info, I'll never be tempted to try aftermarket ones.

I never used anything else than the DJI props but I still get this bumping from time to time. I check the props before every start and there is no scratch on them. I had a very long period of good flights with no bumping in between until it hit me again.

Just wait and let us know when it starts again.

Great info, thanks

I must say I don't use DJI props all the time, right now I have a set of Anbee props with no issues. I have a third party set and I have never had an issue when taking off, no bouncing, no drop in performance what so ever. Just saying...