i have a 1990 mercury cougar its having a weird issue

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i have a 1990 mercury cougar 3.8 v6 its having a weird issue its like it surges when driving at higher speeds you feel a small push pull feeling, also going up hill its like i loose throttle at about 20mph then i feather the gas for a second and it comes back and this repeats at about 40mph or when i i need to pass or speed up in general i took it into my ford dealership they plugged it in told me it was the egr valve the dpfe senor and the tpi sensor wich all have been replaced still having the same issue took it back in he told me it was my altenator and fuel relay switch both replaced brand new as well still having the same issue most of my family is all mechanics and there completely stumped and this didnt come on slowly i noticed it last week on the way back from work o ive also replaced spark plugs properly gapped fuel filters hell replaced the suspension air filter oil change the works and im still confused