710,000 km wtf, are you kidding. does it have original engine and trans.
well i shouldn't be surprised, i have seen mercedes where the odometer rolls over and it still runs good. anyway with such mileage i think there would be a lot of deposits on the egr valves. try removing the valve cover and cleaning it up.
when was the last time you have done a tune up? maybe it's a distributor cap, wires or plugs issue.