I have had a 63' 3020 Diesel and it has run great for a year. Just recently it starts to die after 6 or 7 minutes. It has the clear square fuel filter and starts out completely full. Starts right up and runs. after a few minutes you can see air bubbles forming after about 5 min it gets frothy and starts to sputter. I can let it sit for 2 min and it fills back up and runs great for 5 min again. I have tried bleeding the lines over and over at the fuel pump but no help. I don't see any leaks in the line or fittings. Before I just start replacing lines and fittings can anyone tell me a good place to start looking for where air is getting in. I have a full tank of diesel.