hello from California, USA.

Hopefully Scotty can chime in later but to my ear it sounds like a problem with A/F ratio. Jumping from low speeds to high speeds creates a strain on the engine obviously, but minor faults on the intake gasket and other seals be masked at higher RPMS due to the increased vacuum pressure against the gasket/sealing materials. My best recommendation would be to try a vacuum leak test at idle, WITH THE ENGINE WARMED UP, slowly rev the engine up  and w. the propane enrichment method, test around the fuel injectors and intake gaskets, intake manifold to check for leaks. If you feel that the engine is revving up or misfiring when you point your tube toward a certain spot then you found your faulty  seal to be replaced.

Note! the propane enrichment method is safer than the carb/brake cleaner method that poses more of a fire hazard. Keep an appropriate fire extinguisher handy under any method as a precaution.