I have a 1998 ford mustang with the 3.8l base, I deliver pizza's (sadly) and when i first start the car it runs fine everything is just peachy, but if i run it for about half my shift (4hrs) or sometimes sooner, if i let it cool for anymore than about 20 min it won't stay running unless i throw it in drive and punch that gas pedal for a second, also when i'm driving the car misfires. please any help, if you need any more info just ask.