1999 Ford Mustang runs rough and stalls

My car will run fine for the first 20 to 30 minutes. Then it seems to start misfiring and runing rough. Wont let you go any faster then about 45 mph and the whole time the engine seems to max out and then losse power til it stalls. Then you can sit for about 10 minutes and start all over again. Each time the time it runs for shortens by about 5 minutes or so. It also has thrown code p1506 I have replaced the iac and the tps and the code is still there. Can this code actually cause the problem I have been haveing. And what can I check for.