That's definitely interference, possibly caused by the speed control, or at least the proximity of the ESC to the receiver. Unfortunately, there's no scientific way to find that "smoking gun," to definitively say that such-and-such is the problem, then fix it. It's a trial-and-error process.

You've pretty much elminated the main possible remedies as courses of action, so you've got two left:
1. Add the capacitors, see if that helps.
2. Replace the ESC with another, less questionable brand of ESC.

You could try to disable the BEC, just to see if it works. Carefully pick the center (red) pin out of the servo connector with the tip of an Xacto and tape it back. This way you can try a separate receiver battery, then re-insert the pin later to re-enable the BEC.