Wondered if anyone might be able to help.....

Have owned this car from new and this problem has existed intermittently (annoyingly never while with dealers). Car chugs along as though in too low a gear while increasing speed. Dealership has talked about 'engine regeneration' but this not the issue. Feels like dirt in carb or injection issue.

Also stalls far too easily- once in the middle of a roundabout! Coming to the end of warranty and need problem solved. Any ideas.... please???