198 surges at partial throttle cruise

Distributor bushings aren't causing your problem -- your car has electronic ignition, which is not subject to dwell and timing variation due to sloppy distributor bushings (which at 75k miles you almost certainly do not have).

Quick and easy way to check if the EGR is causing the problem: remove and plug the vacuum hose where it attaches to the EGR valve. Keep your ears open for pinging, though. (And while you are at it, bypass your OSAC valve—that's the widget perched on the passenger side of the firewall, with one hose to the carb and one hose to the distributor. Run hose directly from carb to distributor, not stopping at the OSAC valve, and your car's driveability will improve markedly without any negative side effects.)

It's likely your carburetor is over/due for some attention. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

Additional tune-up parts and technique suggestions in this thread.