TVP5150 losing lock

My company has used the TVP5150 interfaced to the Freescale iMX21 in previous designs with great success.  Our latest design, however, is a totally different story.  The only difference is that we are using both video inputs rather than just one of the inputs.  Some of our production units are exhibiting abnormal behavior when an NTSC signal is applied to one of the video inputs.  We have noticed that the 5150 for the first few minutes after power up is unable to achieve lock (we are reading one of the status registers to determine if H/V lock is achieved).  Then suddenly, the 5150 will achieve lock and work correctly for the remainder of the time it is powered.  In other instances, the 5150 attempts lock at a periodic rate of about once every 1/2 second, but never achieves lock.  If this unit is powered down, and allowed to sit for a few minutes, then re-powered, it will achieve lock on command, and work as expected.  We know the 5150-to-iMX21 interface is designed correctly, as we have 1000's of production units to prove it.  Our 5150 circuit  is basically a copy of the SLEU044 figure 1 schematic except for the following:  no R5 in parallel with the xtal, slightly different LC filter values, iMX21-generated reset, and 75 ohm termination.  Register settings are default except for:  03=0D, (YCbCr, HSYNC, & SCLK out, 04=C0 (unmask all modes), 28=02 (NTSC BT.601).  Any suggestions?