Erratic engine warning lights / limp mode

The car has done 24k but about a year ago started to flash into limp mode (under 3000 revs, no cruise, no stop start) for no good reason. It is under an extended warranty and the dealer has adjusted the crankshaft and replaced some valves but cannot identify what is triggering the fault. It appears when the car is simply toddling along (once at motorway speed it hardly ever does it). Stopping and restarting cures the problem for a while but sometimes leaves the engine light on. Regular diagnostics don’t identify the fault. Anyone have any ideas? The car is otherwise fine, engine timing okay and do on.