Can you use a short micro USB to Lightning cable to connect the Spark RC to an iPhone and, therefore, eliminate the need to establish a wifi connection between the two?  If this is possible it probably be more reliable.

Thanks for comments.

What I did was use an OTG cable and then plugged in the usb to lightning cable.
It worked perfectly with no video lag or loss in connection. (Just make sure your wifi isn't connected to the RC)

You can do that on both Android and iOS, but so far it seems that on iOS it was removed on new beta firmware for RC.

This is what you need http://amzn.to/2v6WicP

I just ordered something similar. I have no problems with my wifi connection to my RC but I thought if I'd order one anyway. It'll be here on Tuesday. I'll let you know how it goes.

I believe the OTG cables for the Mavic Pro can be used. I found them on the DJI Store and there are 2 micro USB and 1 Lightning versions available

I have 1' micro usb to micro usb OTG cable and a micro usb to lightning adapter I ordered from amazon. I just tried it today and it is absolutely a HUGE difference. After connecting cable to remote and iphone, you MUST turn off wi-fi to ensure its connected to cable only. I had 0 screen or A/C lag  ,no compass errors (about 10 times yesterday w/o cable), no switching from atti to gps mode,  no transmission errors, etc. It cleared up everything that was wrong with it. And I didn't have to turn off bluetooth! Now let me be clear, this is aside from the normal errors that you will still get. NOTE: this was my experience, and it went perfect for all 3 batteries