Lexmark T642 gives band of fading

Hi,
I am using Lexmark T642 (Laser printer) for my application. I use this
printer for printing "transparency" like material with transparency
set up.
The problem is (time to time it happens)
1. it prints perfectly OK for few lines
2. it gives a bad of toner fading (about 20 mm)
3. it prints perfectly OK for the rest of the document

When I change the "transparency" like material to paper, the problem
goes away.

Could you please anyone help me? What is this problem? Could it be my
substrate? or some printer hardware?

Thank you for your help.
S