Not really sure how else to troubleshoot this one, so am posting here in hopes of some light on how to solve this.

For a couple of weeks now I've been having this problem that when I power up the quad, the VTX will jump into a random band&channel (although last time it seemed to jump mostly between R8, which is what its set to, and L1 or L2).  I'm not able to replicate this issue on bench, it happens only when I'm on the field (annoyingly).

VTX is a HM OVX306
FC is a CrazyF411 Pro, being powered by 4s lipos

I have plugged the vtx on another quad an it powers up normally on the right channel.
I have plugged another vtx on the quad and again, it powers up normally on the right channel.
To make things worse, the issue is not happening when I'm at home testing it out. I've tried several power cycles and they all powered up no the right band/channel.

I have done continuity checks on all the pads and pins, and also checked the wires. Nothing out of the ordinary. 
Also, I checked both FC and VTX for physical damage anywhere but couldn't find anything broken or loose.

Could the cold weather have something do do with this?  (The temperature has been around 10 Celsius, which I wouldn't think its that cold for shit to not work as they should)

Really am out of ideas here. Anybody has any clue?

Is the power supply for the VTX the same on each quad you have tried?   Is it battery voltage? Or is it FC regulated?  (or do they differ!) What are the Betaflight VTX settings from the FC that's not working in combo with that VTX?

Tks for jumping in BadRaven!

Power supply always from the FC (on the quad that the issue appeared and on the second quad which I tested the VTX to and from), they are small 5v vtxs..

The "problem" now is that I haven't been able to replicate the issue on the bench since every time I try it works as it should.

Its set on SmartAudio, both vtx's are the same model and with the same settings, R8 on 25mw, pitmode off, low power on disarm also off.

Possibly a cold solder joint on the SA wire? Works fine at room temp, but temperamental connection in the cold? When it boots to the wrong frequency in the field, you could try going into BF OSD menu through your goggles and see if you can correctly set the channel.

Tks mstc!

I forgot to mention that I had re-soldered all the wires from the FC already, which doesn't guarantee cold joint free, but I'm sure the SA wire is not cold soldered (paid close attention to that one). Having said that, might redo them again.

When it boots in the wrong frequency, nothing works apart from the video feed. I've tried changing it connected to BF, via OSD menu, and on the TX via the VTX administrator, none did anything. Also the OSD parameters for the VTX always shows according to whats been configured on BF, meaning, whenever the problem occurs it will show its connected to R8 but in fact is transmitting on anything but.

It sounds like the VTX did not boot up properly. I had some problems with a few HM OpenVTX boards in the past, mostly when I used the low power until arm setting (it would on occasion do completely the opposite, power up on boot and go into low power upon arm). Just a wild guess as OpenVTX works with both SA/Tramp there is probably some initial handshake/detection which seems to be failing with that specific FC.
If you have some good reason to keep it on that FC, maybe try another TX pad, but otherwise I would just swap it to the other quad where it works without issue, and put the other vtx that has no issues on this fc.

Yeah, might end up doing that at some point. I want to do at least one more test on the outside to see if the problem persists.
If it does, then I'll proceed to swap the VTX, and if that doesn't solve it, I'll try another UART, and if that doesn't solve it, well, then swap the FC I guess..

Just an update on this one.

I have done a few more tests on the bench and everything was working, tested after several power cycles, on different days and even had the quad sit outside for a bit to get if cold, also tested swapped vtxs with another quad that had the same vtx model. Thought the problem might have been solved, but when I got to the place we fly, same problems happened again.

I have now removed the vtx and put another one altogether, different model and brand, which means I have redone all the vtx wires on the fc. On the bench everything is working fine. Waiting for a chance to test it out on the field now and will post back once I do this.

If it still has the problem, then I'll try to change the UART and try again. If the problem still occurs then it might be something dodgy with the FC and I'll proceed with testing on a different one.