Address that hot fuse box. I'm sure you've seen countless other posts regarding that one specific area of the Z cars.
Z cars are notoriously underwired and one of their major faults is in the fuse box. Once you start having corrosion on the fuse connectors or blowing fuses due to the corrosion on the connectors in the wire harness you are one step away from having major and I mean MAJOR problems.
Look at the posts from other people having problems with alternators, turn signals, headlight switches, tail light switches, heaters, etc etc etc. The list goes on and on, and almost in every case repairing and restoring the contacts to the fuse box helped eliminate or ameliorate some of the problems.
You already know of one circuit that's giving you trouble, could the problem with your surging be due to other circuits also having the same problem?
Take that fuse box out, clean out all the fuse contacts, make sure that they are bent back in such a way as to grip the fuse tightly. The reason Datsun used to include a fuse puller on the fuse box cover is because the fuses shouldn't be easy to pull out. Use a little bit of sand paper or get some steel wool and wrap it around a small screwdriver or toothpick and brighten up those fuse clips. Same goes for your fuses, but first check them with a meter, make sure you have continuity through the fuse, don't just visually check it, many people have reported having bad fuses even though they visually looked ok.
Well that's my 2¢ worth.
