Z Axis G90/G91 Issues (2 replies)

This is my first build involving 3D printing. I am however familiar with CNC milling machines and used to be a Tool and Die maker. So this problem jumped out at me immediately.

I am using Sprinter w/ mech endstops on a taurino knockoff Mega2560. (Which I suspect is my real problem here) but I am looking for an unbiased opinion.

The print will start just fine.

G21 ; set units to millimeters
M107 ; disable fan
M190 S55 ; wait for bed temperature to be reached
M104 S200 ; set temperature
G28 ; home all axes
G90 ; Use Absolute Coordinates
G91 ; Use Relative Coordinates
M109 S200 ; wait for temperature to be reached
G90 ; use absolute coordinates
G92 E0 ; reset extrusion distance
M82 ; use absolute distances for extrusion
G1 F1800.000 E-1.00000 ; retract
G1 Z0.450 F7800.000 ; move to next layer (0) and lift
G92 E0 ; reset extrusion distance
G1 X90.234 Y90.234 ; move to first perimeter point
G1 Z0.250 ; restore layer Z

That last line of G Code is where the problem first shows up. Rather than moving in a (G90 ; use absolute coordinates) as commanded the Z Axis continues to respond as if in Relative Coordinates, and move farther away from the build surface to attempt to lay down the first layer. This continues through out the print resulting in an exponential stretch in the Z axis and distorting the print into a modern art piece.

Does anyone have any ideas as to why this is happening?