I cant get the weed wacker to stay running?

The weed wacker didnt start so I thought it was the sparke plug .I changed it .Still didnt start. I changed the fuel lines,fuel filter and cleaned the air filter. now its tough to get running and then when it is running and runs out of gas or I shut it off it doesnt start anymore until around 2 hours later when I let it sit. What else could be the problem. Keep in mind I had the Jon Deere for 7 years and did little maintenance.