I'm running a XILO Stax F4 Flight Controller + 45A 4 in 1 ESC, and last weekend I was getting low RF signal warnings here and there.  So I decided to move the RX elsewhere.  I have a cap on the main leads that was in the way and I'm running 4S, so I removed the cap. 

Today I went to fly, and any time I did a flip, roll or throttle punch, the quad would just die - video cut out, motors would quit...everything.  Everything would work as long as I just did a smooth hover around. 

Tonight I put the RX back where it was originally and added the cap back on.  Everything runs perfectly again. 

I usually don't run a cap on my 4S builds (although I've been making it a habit the last couple of builds).  Is a cap supposed to make that big a difference, or could there be another problem I'm not seeing?

I wonder if your lipo is just tired and have a lot of voltage sag.

You should not have the issue you are having even without cap.

I tried with multiple packs, all of them freshly charged.  The motors sounded weird too before it happened...normal, but just a bit..."off", I can't describe it.  Even on the bench without props they sounded a bit off.  I tested without props, and the motors and VTX all shut down when I'd give it a quick bit of juice without the cap.  With the cap I can't get it to happen again, and the motors all sound fine.  

Tomorrow I'll try it with the props on, but it really seems like everything is fine.  The whole thing just perplexes me, though.

Hi JinxFPV,

What size Cap is it (uf and volts)?

As voodoo mentioned, it shouldn't make a difference. On the other hand, a capacitor stores energy and effectively "smooths" voltage drops from the battery. So... maybe.

35V 1000uf that came with the kit.

Put the props on this morning and flies smooth as butter again. Really weird.

I also don't have any real explanation but it seems to be another example of why it's a good idea to always fit a capacitor at the initial build stage as an insurance policy unless you have a good reason not to.

The capacitor provides current when the battery sags:

I have 9+ quads
Several 3" ... to 9", 14".

I ALWAYS fly with lowESR, and 35V Panasonic FM on all 6S builds - otherwise motors sound like angry bees, and white lines in video.

Debating $1 lowESR is like debating scotch tape vs quality battery strap. There is no debate.

To augment {and help us to understand} the explanation by TrikeTrash, Pawel Spychalski's YouTube video graphically shows motors generating voltage spikes and how the ESCs mitigate most, but not all, of those spikes.