Add: what kind of sd card are you using? Did you try to format a/o replace it?
I've a Chroma too, if sd card is damaged, too big, not properly formatted or not fast enough, that problem happens.
Also, I don't know the cause, if the battery of the ST10 is low below 50%, the distance of the video connection is reduced.
My Chroma (EU fw) maintains connection at a maximum distance of 250~260 mtrs, it seems that with US fw the distance is much increased.
I did a simple and easy to do mod to improve signal quality, you can see it here: ST10 long Range mod
Another advice is to switch off wifi connection on your mobile phone when flying.