Hello there
well, this should not happen. The HP printers that use the 100 or 102 photo-grey cartridge, like your 8750, will print really first-class B/W prints. I have used one for two years and had excellent results printing only B/W. Are your colour results OK? If so, it is hard to understand what is going on, but I would suggest -
* Reduce the ink load to minimum in the HP driver for the HP glossy papers.
* Make sure that "Enable Advanced Printing Features" is checked OFF in the Printer Properties Advanced tab if running Windows XP.
* Let printer manage output in PS (no profiles).
Send the printer a decent grayscale image at 8 bits, 300 dpi with a stepwedge so you can see where the problems are. Use the HP Premium Plus paper for starters as a known quantity.
John