(Cross posting my solution from another thread)
The EVO 4G seems to have trouble with non-TRRS plugs. I've had all kinds of problems with a standard 3.5mm TRS plug going into the aux of my car stereo. Songs skipping, pausing, voice dialing activated randomly, headphone icon switching from TTRS to TRS intermittently, etc. These usually seem to be caused by jiggling of the plug and/or cable.
I searched all over the web and it seems to be a common problem. Headphone blocker doesn't totally fix it. I've seen folks suggesting trying gold plugs, shielded cables, and various other kinds of voodoo.
IMO the problem is due to a cheap-ass headphone jack in the phone itself.
WARNING!!! DO THIS AT YOUR OWN RISK!!!
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I opened up the phone, removed the tape from the top of the headphone jack and bent the pin on the copper lead that's over the top of the jack more towards the bottom (Away from the S ring and towards the R ring). I've had no problems since.
Your mileage may vary.