Need help with a 1997 Mercury cougar 3.6l

Hello everyone, I could really really use some help with my friends cougar. Not 100% sure what is the issue but I'll explain what is going on. So a couple months ago his vehicle started running really bad. When you first start it, it starts just fine. But as soon as it gets to operating temperature it starts acting like the catalytic converters are plugged. Has really poor acceleration, the engine starts surging, won't go over 30 mph. If you turn the vehicle off for a few minutes and then start it again it will run fine for a couple minutes, but then it starts with the engine surging, poor acceleration, and won't get over 30 mph. Also when you come to a stop at a light or something it eventually stalls even if you are trying to give it has. We know it has some vacuum line issues, but haven't been able to come across a full vacuum diagram for his vehicle. I don't think that the issue is only within the vacuum lines (vacuum leaks), I think there is something else as well causing these issues. Any help or advice would be greatly appreciated. Also if anyone has a full vacuum line diagram for this vehicle they wouldn't mind sending this way I would greatly appreciate it too.