Immense power loss

I have a 1993 SL 3kgt (180k miles), and im seeing major dips in my top speed, (105, previously 140), this has happened quite suddenly and im unsure why. Im almost positive its a bad spark as my exhaust is puffy and uneven. Or maybe a clogged cat. The engine itself was remachined at 160k miles. So I am assuming its either fuel delivery or an exhaust blockage. Any ideas on the best ways to check?