1998 Honda Civic P0133 Code

I have a 1998 Honda Civic CX with approximately 104,800 miles. The car has experienced only usual mechanical wear throughout its life - no accidents or other problems. The exhaust from the cat converter down was replaced at 98000 miles with no problems -old O2 sensors were reused. I recently had the clutch replaced as routine maintenance. Almost immediately after getting the car back it put out a DTC code of P0133. I checked all of the wiring and everything seems to be in order. Simple voltmeter test indicates that the oxygen sensor is working properly. Based on the age of the car and the mileage, I went ahead and replaced the oxygen sensor before (actually at the top of the cat conv - there is another after) the converter anyway (Bosch replacement part). Car seems to be running very smooth, yet the code returns again after a few trips (idled fine for over 30 minutes without generating a fault - then drove it several miles with no problems, after several trips same DTC returns). I have checked wiring and the engine compartment in general and can't find anything. Not sure what else can be done to diagnose the actual cause of the problem.