For starters I believe you should always use google and wikipedia for research. I just spent ~10 minutes to find out more. I'll post what I found. I do not believe the glow plugs themselves change resistance noticeable. I know the voltage across the glow plugs are controlled by the BOSCH controller to perform the pre-heating and after-heating function. For pre-heating you need high power (higher voltage) and for after-heating you need just to maintain high temperature and thus less power (lower voltage).
I'll give you a bit of information which is general to most glow plug applications. This covers the very basics of glow plugs.
Tomorrows technician wrote:
Inside each glow plug is a high resistance heating coil. The plugs are designed to handle high current loads (up to 300 amps in some applications) so they will heat up quickly. Within four seconds or less, most glow plugs on newer vehicles will reach a surface temperature of 850 C (1,562 F), though on some older applications the wait period can be 30 to 60 seconds or more. Glow plugs in a fast-start system take only about four seconds to reach 1,000 C (1,830 F). The higher operating temperature combined with a faster heating rate means less waiting and less cranking to start the engine.
Normally, current to the glow plugs is cut off when the ignition is turned to the start position. Since the late 1990s, glow plugs have been designed to remain operating while the engine is cranking and for a short period of time afterward to keep the fire lit while the engine warms up. The after glow period following startup may range from 20 seconds to a couple of minutes depending on the application, ambient air temperature and engine temperature. When the glow plugs are no longer needed, the system control module or thermo time switch de-energizes the glow plug relay, cutting off power to the glow plugs so they can cool down.
Go here for the full story:
http://www.tomorrowstechnician.com/Arti ... esels.aspx
More information:
http://en.wikipedia.org/wiki/Glow_plugs
Take a look under problems and maybe the glow plugs are "worn" because you live a cold place. Perhaps replacing them will give you the boost you need to get back to good cold starts again.
More here with waveforms:
http://www.picotech.com/auto/glowplug.htm