How much horsspower does your 108 have?
You don't need the red spring in a tractor that has under 20 horsepower and is use for daily use. I would use a blue spring form Cub Cadet Classics
http://www.cubclassics.com/. He is one of our site sponsors. Another option is to use a cultivator spring that was talked about in another topic.
I have a cultivator spring in a IH Cub Cadet 100. The engine is making 14 to 16 horsepower. There is no slipage in the clutch and it is easy on your leg.
Odds are your clutch hanger is flexing....red spring cause the problem. Another area to look at is the area where the throwout bearing rides. That area may have some wear in it.
The clutch adjustment nut may be moving. I have to use a jam nut on my 100 to keep from turning.