'98 C230, impeccably maintained, with 35K miles is exhibiting worsening bouts of poor driveability, with increasing frequency. Occurs only after the engine is fully warmed.

Without warning, the engine will suddenly become poorly responsive to accelerator pedal input - engine just sort of stumbles, then revs, without a corresponding power output. The engine never stalls and the problem typically clears within a minute but can always be cleared by turning off the engine and restarting.

The vehicle has been to the dealer twice in the last six weeks but they state no active or stored fault codes are found and decline to render service until a fault is found. However, the problem's intensity is worsening and has now become a safety issue.

Any thoughts - mass flow sensor? 722.6 transmission control module?

TIA