Loss of power and acceleration

I apologize if this question has been asked and answered... I attempted a search, but don't know enough about volvos yet to even do a decent search. Forgive my ignorance.
Anyhow. I own a 1985 Volvo 240, non-turbo with an automatic transmission.
Much of the time it runs very well, but sometimes it will suffer from severe loss of power and acceleration. I will go about 20 mph, and will refuse to go faster for a bit... if the driver lifts his/her foot off the pedal a bit, it will slowly gain speed.... once it gets up to speed, it drives like a top. It doesn't do this all the time... and it seems to happen without warning, and without regard to weather, temp of car, etc.
I got this car for my wife, and she doesn't want to drive it until it behaves properly all the time... she is afraid it will do this as she's pulling onto the highway, etc.
Plug wires, cap and rotor, have been replaced. The mass airflow sensor looks to be new, and the guy I bought it from told me it was... alternator is new... battery is fine.
Any help would be greatly appreciated. Thank you very much!