Hi all, this my problem. Protege 99 80k miles, AT, 1.6L, when I'm driving around 42 MPH the engine misfires (thats what it feels like)hard for a brief moment. It also happens around 75-85 MPH but not so hard. Could this be a tranny problem? I'm not getting a check engine light. I also notice that the iddlespeed is around 500 RPM which is kinda of low.

The only things that I could think is to change the plug and wires or check for a bad cat because the exhaust sounds like is getting clog. Any ideas? Thanks in advance.