on a 95 E320 with about 119k the car looses power and I hear very muffled "pops" (not exactly the "I'm-being-shot-at" backfires I would expect) when under sudden acceleration or load as in going up a grade. then suddenly, in what I can only describe as the car "catching up," is there a return to normal power and response times. The car idles super smooth too boot. Any idea on here I should start?