91 escort gt idle problems

OK heres the run down
-1991 Escort GT 1.8l
- 250k miles
- Just replaced the alternator and now when the engine is at operating temps, the idle will drop to 500 rpms then catch itself and return to 900 rpms. About a sec later it will start over and drop down to 500 rpms, then back up to 900. It started after I replaced the alternator. I dont know if iis because I had the + battery terminal off for about 3 hrs. Need some clue. This car has been mostly trouble free other than normal replacement parts. A few things that have been recently change with-in the last 5k miles: O2 sensor, wires, cap, rotor, plugs.

It will most likely be your air intake make sure all the little hoses are connected and make sure you don’t have any air leaks around the hose clamps, push the tubing in as far as it will go. Also try the wiring on your o2

Thanks I'll try that

Still need some help, Please