garmin problem with GPS of handheld

I have been having trouble with mine losing collars also. I talked to Garmin and they talked me through some things over the phone first. First I had to unplug the white wire in all the collars for five minutes and plug back in. Didn't make a difference. Then I sent my unit back and they sent me a new one. Got it back and used it last night. Same problem! After 200 yards it lost one dog. Then it would not pick up two dogs for a long period of time. The other dog would go on and off with communication. So the two I lost I tried turning off, deleting, re-adding.. Finally I turned both of them off for about 2 minutes and back on, re-added and they worked. But after about 200-300 yards out, the garmin lost them again. I also used the long range antenna on the system as well. When my Garmin was in the shop I used a buddys and it picked up my collars fine. So I dont understand the problem. Has this happened to anyone else?