Car gets up to 40 mph then slows down to 25 mph on 2001 Nissan Sentra

The car will get up to 40 mph then will slow back down to 25 mph. Changed out wires, plugs, distribiture, cleand fuel injectors.

Asked by for the 2001 Nissan Sentra
If is hard to accel up to that speed and drops quickly then may be restricted exhaust possibly the cat conv. Have a shop check it out. Maybe cam timing is off.