Maybe ultrasonic cleaning the contacts and deoxit gold?
Are you using high end VNA cables? I do wonder if they are the best type of cable for low resistance measurement, i.e. they have foam dielectric in them.. I only have 18GHz or more cables but they are thin and I don't have the parts for making the PCB yet unless I want to do it with waveguide launcher connectors (I found like 100 sma waveguide launchers in the trash a long time ago).
Now I am not sure what dielectric hard line solid sheath uses, but I have a feeling for that kind of stability you don't want foam in the circuit, and for some reason I think a higher wire gauge conductor is better.
I do have some hard line however, if I do this I could try to make BNC hardlines that go to the PCB if you feel like waiting a month. I think my rg141 is solid core and solid teflon dielectric
I need RG400 or whatever its called for my pogo pin fixture, the brass tubes should get here but when I think about it I think I need to redo the plan with a peice of hardline rather then a flex cable because mechanically some kind of bullshit will go down with anything but solid teflon insulation
Also, if you make your own cables, since it turns out bondable (not sure, still need to experiment), you can try to glue the pin into the connector after soldering or crimping to the center conductor by using a primer and loctite glue (medical) to bond the center pin to the connector with more then press fit, i suspect as you are bending the cable the center pin is moving around inside of that plastic sleeve and causing a problem
of course what i recommend is going to go into full hardline for the fixture that is well bonded into place some how (maybe stainless zip ties on a metal frame) that goes to the VNA and then use all APC-7 connectors and retrofit the VNA with APC-7 so it can be put into exactly the same space every time and connected with genderless rotational sleeves so you can put it into a fixture bonded to the analyzer every time then tighten the sleeves without fiddling with anything, but that is like a 1000$ mod, because if you use bigender/genderless (what is PC now lol?) connectors you can put the fixture on machine pins and then do the same thing every single time. I am imagining a 1/4 inch plate or better bolted to the bottom of the VNA with pins on it to fit the fixtures*
*its been a while since I used APC-7 connectors so I am not sure they are truely machine pin compatible, because you might need to tilt them, don't do this unless you figure out a connector that will work, I don't feel like looking for one to see how flat the face is right now, or if there is any spring loaded stuff that you don't want rubbing on a slide fit in there. I think it can work if that is a problem if you put the pins orthaganol to the plate so the fixture slides on from the front rather then slides to sit on top of the plate, or you bend them, but I feel like hard line would be best left as strait as possible without bends introduced into it, ready made coupler/extensions might be the best to use