Z Over Travel with -7.13 Tool Offset

I was profiling a plate clamped to the table (top surface 0.75" above the table). It made one pass 0.050" then hit a 318 Z Over Travel. I changed to a long nose holder so the offset was about 1" higher. As soon as the Z1.0 move was started in the program the 318 error happened again. After trying different offset values I found whenever the tool offset is anything greater than -7.13 the 318 error occurs. Above that value the spindle travels down until is hits the Z stop.
I tried re-homing and power down restart but the problem still occurs.
I can manually move the tool down to where it is below the table surface. I checked the Z limit steps in the parameters and found it was set to the same value as the Y axis. I believe it should be the same as the X axis (20" travel). But that is not the cause of the 318 error.
Any ideas? '91 VF-1

did you update the H value?

Post the chunk of code where its happening. The control reads a few lines ahead into the buffer.
It won't necessarily alarm on the bad line, it could be that it read a bad line into the buffer.

Is there a z value left over in your fixture offset from the last job? That screws me up sometimes.

Not a program or tool length offset problem. Today I tried just the countersink, which worked before this, and it also came up with the 318 error when the line with the Z move was encountered.
When the machine booted it had a motor interface board error which cleared with the reset. It appears something went south there.

Well that stinks.

Made more tests and it may not be the Motor Interface Board.
I swapped the X-Z encoder inputs and X-Z outputs on the MOTIF board and also the X and Y servo cables then ran the countersink program. The X and Z positions going into the processor should be the same but using different paths through the MOTIF board. Same 318 Z error. If it was a chip in the Z axis path on the MOTIF board I would have expected an X over travel error.
Now, the weird part:
If I touch off the tool and set the offset, then reduce the steps/inch in the parameters to half the normal value, the tool will go down to the correct depth without the 318 Z error and do the drilling routine at the depth where the 318 error occurred.
Of course, if I touch off the tool with the half step/inch value the position page shows twice the distance as expected.
Hand cranking the Z axis with half the steps/inch moves the spindle half as far.
Since the steps/inch value was changed after touching off the tool and before the program was run, the program must not be using the steps/inch for position, just travel limit.
My friend thinks it might be the cable or encoder.

Well, stupid error. Voltran had it right. The G54 Z value was way off. Normally I have this at -0.5 because I set my tools with a 1/2" pin. Somehow when setting the X and Y values I accidentally changed the Z to around -14.6.