1994 F-250 Computer Problems

Looking for info on what to do about my truck. My '94 F250 has a 7.3L 460.
103k and a good clean engine with no oil leaks, generally a really good
truck. It began to run like crud so on the recommendation of my Mechanic
I replaced all of the EGR (4,metal ones off back of engine) did an
extensive tune up, replaced a bag O2 sensor and thought I had fixed the
problem of awful gas milege (5mph), running rough and small exhaust leaks.
The truck was running excelent with no exhaust smell, smooth idle, lots of
power and 10mpg in the city upwards of 14mpg on highway.

now 1500 miles later, the truck is idling rough, I get the smell of
exhaust again and the milege is back to 100 miles to 20 gallons.

I was told that this year model has a computer problem and that my
computer is telling my truck to run like crap.
Dose this sound realistic and are their options shy of a whole new
computer system. any ideas would be appreciated.

Thanks