HP 5si print quality problem

I got this HP 5si printer for free the other day but have some print quality problems which I would like to diagnose. When printing text the whole page has a light gray shade on the text side (and the first page generally displays the same on the back side) and when printing a complete black sheet the black is not solid black but has what I would describe as a plaid pattern which makes it dark gray.

I stopped the printout to see where in the process it happens and found that the page is that way after the toner cartridge and before the fuser. The fuser does not make it worse.

I have not yet tried a new toner cartridge (which is probably around 1-2 years old and was not used much, if at all, during that time). Did anybody ever run into this problem?

Besides the obvious, could it also be a problem with the laser/optics and if so could that be fixed and at what approximate cost?

Thanks for the help.