The flange is about 3/8 above the floor and the toilet rocks side to side. How can I fix this. There is very little room to cut the cpvc to install another flange. Any suggestions are appreciated.

Usually, most of today's toilets will absorb 3/8" of height of closet ring. There is a chance that the toilet is rocking becasue the bolts are not solidly attached to the closet ring ( and closet ring should be solidly attached to the floor ), rocking with the bowl as result.

Get 1/4" x 3" long bolts, drilll hole into the concrete slab where bolts should be ( on each side of the closer ring ), set bolts into epoxy, allow to dry, install was ring, set toilet over the bolts - straight on, push on it so it compresses wax ring. Now the entire toilet bowl base should be touching floor. Start carefully tightening nuts. If you feel any wobble, use plastic wedges and insert those for suport. Caulk along the base. Allow to set over night.

Don't try to attach ring itself if it sits 3/8" above the floor or you will bent it...