My 93 Sedan Deville runs rough and loud when it's raining heavily. I replaced the plugs with new platinums. Any thoughts as to what may be causing this?

Did it start acting up before or after you replaced the plugs?

If it was before, I would rather suspect wires leaking voltage to the ground when the relative humidity is high. Find some very dark place and spray some water on wires while engine is running. Watch for arcing.