198 surges at partial throttle cruise

Lack of Vacuum advance,EDITas in busted diaphragm,or failed vacuum circuit;
or problem in the low speed circuit, specifically; the transfer slots,the idle bleeds, or the emulsion tubes.

There is also a slim chance that the total timing is over-advanced, that is to say that the combination of initial plus centrifugal,plus v-can, is too much.