If those 3396's are cooked internally then they're done. My best answer for the failure would be that something else upstream in the circuitry is wearing out possibly causing stress and eventual failure on them. ...just a guess without a whole lot of basis at this point. The CEM 3396 is still a pretty cheap IC to replace and available from a few Ebay sellers for as cheap as $5 or $6 a piece, not much compared to a CEM 3310, 3320 or 3360, etc. that can go from as low as $40 to $50 on up.