I had this problem with 1.2Ghz gear for some reason, couldnt get more than 20ft before loss of picture. My advice is leave the VTX set on a channel & put your craft a few metres away, then walk about 50ft+ away & with the VRX, keep changing channels to see if they make any difference. I have a 5.8GHz set that is seriously wrong, I need to be on a different RX channel to get a good pic! these cheap VTX's do blow the power o/p transistor very easily!
Do you have a UHF scanner? I have one that goes up to 1.5GHz, so its always handy to see exactly what frequency your gear is actually transmitting on.
Ross