Slipping - Most likely the teeth on the gears have literally chewed down, this occurs when the side plate screws have loosened enough for the gears to shift out of position, causing the teeth to mesh incorrectly.
Grinding - check the teeth for damage (see slipping - above). If they are fine, sometimes you might just need to slip a shim onto the main gear. We don't have specific numbers for shims to main gears at this time.
NOTE: When greasing gears, do not overgrease them. It is not necessary to pack the housing with grease, as it can actually cause problems. See notes on maintence for more info on greasing reels.