Powerless Trans Am

Hello; I have a question regarding my 1985 Pontiac trans am with the 5.0 h.O. 'F' motor.I have been scratching my head bald with this for the last month. The vehicle is stock except for an Accel coil, wires and bosch plugs, it has over 180,000 miles I assume since I bought the car two years ago. The problem that I have encountered is that as of a month ago, while driving the car all of a sudden lost power. The motor runs fine at idle, no tapping no backfiring nothing at all. But when I step on the gas the car just feels as if its suffocating, backfiring, and slows down the more that I give it gas. When I was on the local road, I gave it gas so as to reach the towns 30 mph, as I did that the car bogged down and it felt as if someone had attached a bungee cord to my rear bumper, just as if you would flood the motor, but once I backed off the gas pedal the car returned to a good idle and ran on its own power at 10 mph without any problems. When I popped the hood and pulled the gas cable the car again backfired and you could see the air intake collapse under suction.I was forced to drive home with my 4-ways at 20 mph just slightly tapping the gas pedal so as not to stall. I also noticed that when I left the car in my garage foe that month and restarted the car ran fine even when I reved the motor high, but once I had the car run for 25 minutes and reved the motor again it began to backfire and sputter. Any help would be greatly appreciated; Thank you