May be these questions will help you.
1.Are the coverage area for both the operators same,I mean is there any “no coverage zone” for the other operator.
2.Are all BCCH frequencies there in the operators BA list?
3.Does the MS got hanged while measurements?
4.Are both the handset models same?Pix has a valid point,I think.
I’ll not say anything about neighbours as you are talking of Idle mode drive.
One more point,Though MS makes mesurements every 5 seconds,it sends the measurements only if a valid BSIC is decoded.