lights flickering and surging

may be a voltage stability problem, have you had the battery tested, checked alternator out put and checked all main electrical terminals are clean and tight?

To me it definitely sounds like a voltage problem. GM's are bad about voltage regulators that quit working correctly when warmed up. And their regulators are built in to the alternator, meaning you have to either rebuild or replace the alternator. Given its age, I would replace it rather than rebuild. Definitely have the alternator and battery tested and see what is what. AutoZone, Advance Auto, and O'Reilly all test batteries and alternators for free and it only takes about five minutes.

And as suggested above, make sure that the terminals are super clean and tight enough that you simply cannot move the terminals by hand.

Thanks for the backup Harry, sometimes working from the other side of the world you do not get to know all the known problems of your cars that we do not see, always good to get some local know how on board. Cheers, Mark in Australia.

Thank you all for the help. I fixed it today one of the wires running to the crankshaft sensor was starting to break apart.

Ah must have been the lead wire. Either way, we are glad you got it solved. Please feel free to come back any time.