198 surges at partial throttle cruise

If it runs smoothly under hard throttle and at lower RPM's (like taking off from a stop), then the fuel delivery is likely A-OK.

I would be suspecting:
- The carb fuel level... the float setting may be off or the float a bit gas logged.
- EGR sticking open
- Weak spark due to a coil or ballast issue, plug wires, rotor, etc.
- Timing variations as suggested