2002 2500DH Voltage Drop

Need some help. My 2002 Sierra HD (Duramax) has low voltage & dim lights @ startup. The dealer claims that this is normal due to the voltage required to heat the glow plugs. Also when I release the accelerator the voltage will drop from 14+ to approximately 11 volts, the lights dim & then it rebounds within a few seconds. Is this normal? Does anyone else have this issue? If anyone had these issues & they were repaired, how were they corrected. Any support is greatly appreciated, since the warranty is due to expire soon.

Thanks!