I have had this happen when using outlets at the track that are too far away from the sub-panel. I have made it a point to setup my pit at the table closest to the sub-panel for this very reason

I suggest trying an outlet that is on a dedicated line without sharing load with anything else and making sure you don't have any extension cords or power strips that might increase resistance.

I'd say more likely is that your iron's temperature setting has been bumped, the iron isn't tinned or doesn't have a clean tip.

It is also possible that the ceramic heating element in the iron has cracked if it got dropped or bumped during your move.

I've had good luck soldering with a cheap iron in the lower mainland so I'd say it is not to do with the humidity or outdoor temperature.

Could also be a switch in solder? Lead free stuff doesn't always look as shiny as the lead stuff.