Yes,
I have something similar. I'm using a 8dB omni dipole aerial on the camera and I think about 12 dB yagi on the receiver. We still get sync loss when something crosses tha signal path. It seems the AGC in the receiver cannot keep up. We certainly have planty of gain, the thing will work forever in a clear line of sight or even with a few walls in the way. I'm pretty certain it's just the change that causes the problem.
How to fix? Well better gear costs a LOT more and also I think uses mpeg-2 encoding / decoding which leads to an interesting problem, there's a quite a few frames delay. This isue came to light during the Sydney Olympics.

With the gear that you've got try to get the Rx aerial up high so nothing gets between it and the Tx aerial. You could make a simple rig to get the Tx aerial up higher, a bit of plastic conduit would do. If the cameras mobile still easy enough. We put the Rx aerial onto a cheap tripod, add a cheap gun site so you can keep the thing lined up or add a cheap laser pointer.
Hope this helps.
Bob.