A couple of comments here. These cars are full of fairly simple (by 21st century standards)
circuit assemblies than control various things - the cuise control amp, heater gubbins,
and in this case fuel pump relay.

A common failure is simply a mechanical one and it's usually at a solder joint,
which is why the oft recommended technique of resoldering EVERYHING on the
circuit board works so very very often.

Tiger is right in that a new relay will most probably fix it. By doing what you've
done so far (fixing a couple of trouble spots) you have almost certainly identified
the source of the problem, because by fiddling with this thing you've at least
changed the nature of the problem and at best have fixed it.

Say you have it fail again and replace it with a new relay. Now, that old one
sits there on your bench with one or maybe two mechanical problems preventing it
from working good as new. We're not talking a mechanical failure here such
as a worn cam but simply some place where the electrons get stuck - sometimes.

That it works at all means the PIC and firmware works, it's just not getting
the data it needs as sostensibly some wire has a broken connection.

If indeed it does act up again then reflow ALL the solder joints. And it woudlnt
hurt to go over it with an ohmeter first to see if you can identify any problematic
areas of electrical discontinuety.

It does sound like you're on the right path though.