ive got a dna 50 and after about 20 mins driving it slows and misses .this becomes worse till it wont top 20 kph or it just cuts out. then i have to wait for it to cool. but it is only at four bars on the temp gauge when it statrs to cut ant the oil and coolant are full. it doesnt like starting when its just been turned off like at a petrol station. starts first time from cold.any advice appreciated