Does code require the gas line to be replaced when replacing a furnace

I'd have a look at your draft inducer motor. Do a Google image search for "Furnace draft inducer" to see what it looks like. I has the same kind of impeller as your main blower but on a smaller scale. You may have some debris in there.

There are a lot of scenarios that can cause a pressure switch to report an error. As an example, one that I have seen that is actually "self-healing", I have seen really cold days where there are ice crystals in the air and as the air is pulled into the air intake it can create an 'ice dam' and thus restrict the air flow enough that the pressure switch does not close. This can melt if the outside temp gets warm enough and it can also be dislodged simply by shaking the intake pipe. I have also seen birds nests and various debris block the air flow. One time I am pretty sure that the wind was strong enough and blowing in just the right direction that it was preventing the switch form staying closed. (I made this assumption because of the homeowners description of his trying to fix it and their location at the mouth of a canyon and the strong winds we had had. You mentioned power cycling the unit. That would reset the error and allow for it to try to come on again. If it is running again it should be good the switch is open and will not run unless all is well.

As far as preventative maintenance, you have done most of what can be. I have seen a pressure switch that had water in it because one of the hoses was kinked. But there is not much else that can be done.