These cars typically burn up the coils and ignition module, we have done several in the shop where I work. The valve covers leak oil and then these ignition components get oil soaked and fail.
This is a pricey repair, like $1800-2000, with the valve cover work, which must be done. The engine in your Caddy is a SAAB motor which is VERY high maintenance.
I was not aware that Caddys and Saabs share the same engine. Although, I do know that 3.0L Saab V-6 is a GM engine that Saab did minor adjustments to(at least that's what they told us at Saab FTP). Saab used a slightly thicker head gasket to lower the compression, added a turbo, altered the intake and exhaust manifolds, and added Direct Ignition with a "Trionic" ECU.