Transmission problems

I'm having trouble with the transmission on my wifes 96 Geo Metro with the 4 cylinder engine. When the engine gets warmed up the transmission takes a lot longer to shift from one gear to the next. Also from park to reverse it takes longer and sometimes I have to rev the engine to get it to go into gear. One more thing please, the Service Engine Soon light is on.
I've changed the engine and transmission oil, plugs, cap and rotor.
Please help soon. I gotta get this thing fixed.