RE: spur gears ?????
By clutch holder, I guess you mean the screw and spring that hold the slipper clutch and spur gear together. Go ahead and crank it down all the way and loosen it by a half turn as the book reccomends.
If you are loosing screws out of your tranny (namely #Z567's), then you need to replace the tranny case. Go to your local RC store (or go on-line) and order a new Savage tranny case. Once you get the case, take the old tranny out, dissassemble it, and begin to transplant the guts (tranny gears, bearings, etc.) into the new case.
While you have your tranny taken apart, inspect all the gears and bearings and lube/replace as necessary. From your description of your spur gears failing too often, it may be that your input shaft (the shaft that the slipper clutch assembly screws onto) may be bent. This will be obvious even before you remove your old tranny. Spin the spur while it is still mounted on the tranny and if it wobbles, the shaft is bent and/or other pieces of the slipper clutch assembly need to be replaced.