Gradually stops after staying at that speed, or after accelerating past that speed? (Go, Speed Racer, goooooooo)
First guess would be tire tread or composition. Are they new tires, old tires, in-between tires?
Also I recently replaced my front stabilizer bar bushings because they were beyond shot. Changed my whole driving experience. And quite easy to do, in addition. Either of these two things could be the culprit. Beyond that, no idea, sorry.