1986 Mustang 5.0 wont stay running very long with loss of power

I have a 1986 Mustang 5.0 5 speed and when i crank it it wont stay running long and i have to gas it to keep it running...it also makes a ticking noise when i give it fuel and it sounds like a stun gun and you can see alot of fumes coming out the pipes and smell it too. It started one day driving on I-95 and i had it on cruise control...then when i had to slow down and accelerate to get back to speed it was losing power and could smell exhaust fumes. The only way to get it up to the speed limit i had to barely give it fuel to speed up...if i gave it too much it would lose power and sputter, then after maybe fifteen minutes it would lose power until i was doing maybe 20 miles per hour and after maybe 5 or 6 minutes it would gain power but only enough to get back to the speed limit. This car has a stock 5.0 HO and off-road pipes and flowmasters. I just bought the car maybe 2 months ago and little by little it seemed it didnt wanna make too much power over 4000 rpm and slowly but surely as time went that rpm number went lower and lower until it seemed it wasnt making anymore power over 3000 rpm and then that I-95 experience happened..