1995 Nissan Sentra rev/RPM limiter

I am a shade tree mechanic but this problem has me baffled. My wife comes home the other day and says when she left work the car barely wanted to crank over so I am thinking it’s the battery since I replaced the alternator 2 years prior. I get my volt meter out and check the batter and sure enough 11.75 volts then I start the car and it the same 11.75 voltage so I go to check my alternator and notice it’s belt was gone so I replaced the belt which connects the Alternator, A/C Compressor & Crank then start the car and now I am getting 13.5 volts which is great but now a new problem as emerged. At an idle (aprox. 800 rpm’s) there is no trouble and the car idles fine but if you try to give it the least amount of gas (around 1500 rpm’s or above) the car rages up and down on its RPM'S when you are keeping the accelerator constant, sounds like the rev/rpm limiter is kicking in why to early. Check the fuel supply and its PSI is constant and accurate. There is no codes and no prior history with the symptom with this car. I wouldn’t think the electrical ignition module, the pick-up coil or O2 sensor could go out just by replacing an Alt. Belt. Any thoughts would greatly help.