The cold-engine idle is a bit high, I'm not sure how the timing advance is done on those cars (I have a diesel) but I'd be looking at that, or what might affect it. Also be a good idea to start the engine at night with the bonnet up and - without a torch - look over the engine, paying very particular attention to the HT leads. Arcing from the leads will most definitely cause issues with the engine.
Surging on the diesels is often due to a dirty SCV, and is often fixed (or at least reduced in severity) by throwing a bottle of injector cleaner in the tank. We use Liqui Moly Fuel System Treatment which also works in petrol, it's proven to work for me in the past and I'll use it again I'm sure. You may not have the same fuel system (and certainly, your fuel pump won't be producing 24,000psi) but there's a chance that this will help so for a minimal cost you might solve it.