Failed autoland on PMDG 737NGX at KSNA ILS19R, rapid descent just prior to reaching runway

I have flown the flightpath from KOAK to KSNA on the PMDG 737NGX prior without any issues using the autoland on ILS 19R.  However recently I have been having problems with the autoland functionality. In my case just prior to reaching the runway, the aircraft will rapidly descend and my display goes from "Land 3" to "no autoland/apch warn".  I found a similiar problem described here http://forum.avsim.net/topic/441447-autoland-disaster/.

I am trying to ascertain if this is a FSX bug or am I doing something wrong that I must be forgetting from the past successful landings.  Below is some information on my setup:

[Procedures]

* FSX

* PMDG 737-700 NGX

* AIRAC 1407

I am flying from KOAK to KSNA during my descent into the Los Angeles region, I perform the following:

* Set both nav radios to KSNA ILS 19R frequency

* Set the HGS runway elevation to 54 and runway length to 5701 I got these values from the IFR plate for 19R

* I set both course headings for the Captain and FO to 194

* I have set a 30 degree approach in the FMC

As I approach SNAKE (int) I am at an altitude of around 3600, when I am turning to align with the runway I press the VOR/LOC button once I am facing the airport I press the APP to lock on to the glidescope, shortly after this I press CMD B and both channels are active. I then set the MCP speed to the flaps 30 which is somwhere between 133-135.

During this time I am extending my flaps at the correct interval when I get to flaps 15 I drop my gear, as I get close to the runway everything is great speed is good and I am at flaps 30. However just prior to reaching the runwayr, the plane will descend quickly and "no autoland" will be displayed. It should be noted that it is not stalling as it appears the autopilot is adjuting power and dropping the nose to get lower.  If i continue on that course it will drop the plane a few 300-400 feet in front of the runway.  To work around this I simply shutdown autopilot all together and take the aircraft in myself, but I have in the past used the autoland feature on ILS 19R without issue.

[system Info]

It should be noted that I do have a dated system, on average I may only get between 8-12 fps when in these approaches so while frustrating, that could be contributing to the issues.  FSX is CPU bound too, so any interrupts will cause every so slight pauses which could be contributing to the issue too. However I was able to complete these approaches using the same system in the past. My installation though has been redone.

Is there anything specific about 19R that I should know about, that maybe is a bug in FSX?  Is it possible that the system itself could be contributing to the problems as well?  Thanks again for any help and advice.

Best Regards,