My 99 crown vic has me stumped help please?

i have a 99 crown vic that sputters then eventually stalls when the a/c is on or im trying to go up a hill at a cruise speed i have replaced the fuel filter and checked the tps and maf and the are working also the check engine light is not on and i have done a tune up about 8 months ago . after it stalls if i put it in neutral and shut it off then restart it it runs just fine for a few miles then starts acting up again