sometimes wipers wont start when turned on. when working it stops at times

The most common cause for the wipers to not work at times on your 1998 GMC Safari would be a faulty wiper motor circuit board. GM had several recalls and policy adjustment on these circuit boards. Please check with your local GM dealer to see if GM will take care of the repair.

Ground your wiper motor to the vehicle frame with a grounding strap or heavy wire. Worked for me