GCode Issue

I have an issue when my print is done.  The Z coordinate does not go to the Z coordinate that I want it to go to.  This at times has caused the z access to go too low and cause some nasty noise since it is driving it to low (resulting in me quickly shutting it off).  I am wondering if this has something to do with the use of G90 and G91.  I believe I adopted these from Luc's script he uses on repetier.  The intent is to lower the bed so I can take the print off, much like stock firmware.

So, any thoughts or recommendations to make sure the bed goes down to the exact level I want it to?

Thanks

Mark

I am using the following G-Code in my starting and ending Scripts:

Starting:
G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
G28 ; Home extruder
G1 Z15.0 ;move the platform down 15mm
G92 E0 ;zero the extruded length
G1 F200 E3 ;extrude 3mm of feed stock
G92 E0 ;zero the extruded length again
M100
G4 P2
;Indiana
M300 S1318 P240
M300 S0 P120
M300 S1396 P120
M300 S1567 P120
M300 S0 P120
M300 S2093 P960
M300 S0 P120
M300 S1174 P240
M300 S0 P120
M300 S1318 P120
M300 S1396 P960
M300 S0 P240
M300 S1567 P240
M300 S0 P120
M300 S880 P120
M300 S987 P120
M300 S0 P120
M300 S2793 P960
M300 S0 P240
M300 S880 P240
M300 S0 P120
M300 S987 P120
M300 S2093 P480
M300 S2349 P480
M300 S2637 P480

Ending:
G91 ;relative positioning
G1 E-1 F300 ;retract the filament a bit before lifting the nozzle to release some of the pressure
G1 Z+0.5 E-5 X-20 Y-20 F150 ;move Z up a bit and retract filament even more
M100
G28 X0 Y0 ;move X/Y to min endstops so the head is out of the way
M117 Cleaning
M100 ; Cleaning
G1 Z170 ; lower bed
M84 ;steppers off
G90 ;absolute positioning
M104 S0 T0 ;extruder heater off
M104 S0 T1 ;extruder heater off
M140 S0 ;heated bed heater off (if you have it)
;FunkyTown
M300 S2093 P240
M300 S2093 P240
M300 S932 P240
M300 S2093 P240
M300 S0 P240
M300 S1567 P240
M300 S0 P240
M300 S1567 P240
M300 S2093 P240
M300 S2793 P240
M300 S2637 P240
M300 S2093 P240
M300 S0 P960