2000 v6 sonata

Not sure what the problem is as of yet, but i have found that the car seems to rev up for longer then what seems normal then when the car is going about 80 kmh the rpm seems to drop down to almost nothing. car has been stalling or almost stalling. had the transmission replaced. took the car in to have it scaned because check engine light is on and they could not communicate with the car. taking it back in this week hopefuly they will be able to solve the problem.

check the battery, and altenator...
plugs wires, oil, crank sensor,

jus as a start

i am going to have a electrician go over it tomorrow. hopefuly he will be able to solve it. thanks for the help though.