[blueosprey90]

Living in the colonies (and having no experience with Volvo B18 or B20 engines), the diagnostic description of "lumpy tickover" has little meaning for me.

But I have difficulty imagining that use of a B20 head itself would result in a "lumpy tickover". The car may have a lower compression ratio because of an oversized combustion chamber, but it would just be down on power.

You describe that the engine was rebuilt, so my question is: what are the characteristics of the camshaft that was utilized. A performance camshaft may result in a loping idle and poor performance in the lower portion of the power band. As you pull away from a standing start, the car may buck and hesitate until you get into the camshaft's power range.

Another option is that mice have made a nest in your exhaust system, with the result that you have too much back pressure. That too might cause something that feels like an internal backfire back into the engine. Not exactly the same an a misfire. Perhaps a restrictive exhaust manifold would result in the same or similar symptoms.

The third option is that you may have sticking valves. If the valves are sticking because of inadequate clearances, it might feel like fuel starvation or a misfire at higher RPMs (when the engine is hot). Often confused for both ignition and carburetor misfire.

Do let us know what you do and how it addresses your issues.