Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I think you might need to do a full uninstall-reinstall of the drivers for this printer on your computer. If you would like instructions on how to do so, please state (in the Reply To Expert function) what your computer is, a Windows PC or an Apple computer, and also what version of the OS is on the computer.
If the problem is in the printer not the computer, then you have a problem either in the way the printer's memory is storing the line to be printed, before printing it, or else a firing problem in the printhead. Or perhaps a varying voltage in the power supply of the printer, though I doubt it.
How old is your printer?
What program are you printing from that has these irregularly bolded lines?
Have you tried printing from a simple application, e.g. Notepad (or TextEdit on an Apple computer), to see if the problem is local to the program you are printing from most of the time?