Mercedes Benz repair: 1985 Mercedes 380SL, pipe sound, exhaust pipe

Question
Car has no acceleration.  Light surges. Worse when AC turned on. Top speed approx 50 mph. Runs a little better after about 15-20 min. Have to floor it just to move...sucking gas....Replaced fuel pump and air filter, but didnt' help...Any ideas greatly appreciated.

Thanks

Answer
Need more data.
#1. How many miles on vehicle?
#2. Do you drive this car every day?
#3. Sucking gas = what MPG?
#4. Is this a California car?
#5. How many owners?
#6. Does the exhaust pipe sound like a plugged up vacuum cleaner under engine load?
#7. Has the ignition coil ever been replaced?
#8. Did the car set for over three months during the last year?