Go to Radio Shack and buy some DeoxIT - Catalog # 64-4338.
Disassemble the mic and spray some into the actual PTT switch.
I had the same problem and did this to mine about 3 years ago & it's been OK ever since.
From: km6xx <firstname.lastname@example.org>
Sent: Fri, June 4, 2010 2:26:43 AM
Subject: [IC-7000] Mic sometimes does not key up radio
I've had my IC 7000 for about 6 months now. Its installed as a mobile with the remote head option. Just this last week, the radio started to not respond to PTT on the hand held mic. Or it would drop TX mid transmission. I tried plugging the mic directly into the mic port on the back of the radio, but I had the same results. I've tried wiggling the cable and modular connectors while pressing the PTT, but I have not had any consistent repeatable results.
Has anyone else had this problem?