Dilantin good, need a little something extra...?

I have been taking Dilantin for many years (let's say 20). It has done very well for me, but I still have 2-4 tonic-clonic seizures a year, just enough to prevent me from driving most of the time.

I would like to consider an add-on drug that might fill the gap; preferably cheap/generic with not too many side effects (I know, sounds like a miracle drug...but I do tolerate most things well.)

I have tried changing to others...Keppra, Lamictal, Zonegran....but they had no noticeable effect when my Dilantin was reduced.

Any suggestions?