1993 Volvo 940

I have had this car for nine years and just til last December it was my daily driver. I was given a promotion and recieved a company car. The volvo has been sitting for awhile and I decide to take it out and drive it this weekend. It was a little slow to get started, but after I let it run for about 15 mintues I headed out. The car ran fine threw town, but when I got to the highway it would only max out at 60 MPH. Before it would run great at 70 MPH and never break-a-sweat. Now it is barley making it to 60 MPH. I was wondering what might be wrong with it. Could a simple tune-up be the trick(change the sparkplugs, rotor, and battery?)The car has never had a lot of get up and go from a stand still because its a 4 Cycle and its so heavy. I just hope that its not a sign of the car final dieing. Any info would be very helpful.
Thanks