Since you swapped HPWCs and assuming you have proper gauge wire, the issue is most likely in the circuit between the breaker box and the 14-50 socket. The HPWC is sensing a voltage drop and is throttling to prevent overheating.
Yes, the terminals in a junction box can cause this. If the j-box is large enough, consider replacing any conventional screw type splices with Burndy Unitap BISR2 (available on Amazon).
Something that hasn't been mentioned, but this problem can also be outside your house in the feed or at the transformer. If it's undersized or failing in some way, you can get voltage fluctuations that trigger the car to reduce power. I would especially be suspicious of this if the voltage stays fairly stable as the car ramps up, and the lowered amperage gets triggered later. Also, if you manually dial down the amperage, but the car continues to trigger and lower it further, anyway.
When I first had this issue, SCE came and checked. They rerun the wire and connected directly to the transformer. It is possible that things have changed. It is really fluctuate. Sometimes it is charging fine but I always have issues at least once every two weeks (that I know of).
