Batboard

Hello Everybody:
I have a GP300 wide band vhf portable model number P93YPC20A2AA that when you first turn it on actualy transmits for about 4 or 5 seconds then stops transmitting .
Another thing it does is when you transmit and let off of the button it doesnt stop transmitting and does 4 or 5 seconds again then it decides to stop transmitting .
This radio has started transmitting all by itself at times and if you turn it off then back on it transmts for the 4 0r 5 seconds then doesnt ket up for many days.
When it does transmit or recieve it sounds fine.
So i took the radio apart and looked at the transmit button = clicks on and off and feels ok in operation =so i dont think its a sticky button.
Put the radio on a power supply at 7.5 volts = same long transmit .
Gave up and left the radio on the table disassembled with metal clips covers off and picked it up 12 hours later only to find it works perfectly after reassembly.
This radio has done this to me about three times in say a couple of months.
Working now but i know its going to do it again cause i didnt do anything to make me think i fixed something.
Anybody got any ideas or had this type of problem ?
What can i do to fix it.
I do have a service monitor available to me if i need it.
It does transmit 5 watts and draws sonmething like 1 to 2 amps when transmitting as i remeber.
Thank you for your time and help.
Satelite

Dirt, moisture, or corrosion in the external speaker/mic jack or under the PTT popel disk.

Bad antenna, bad/bent antenna contact, or incorrect range antenna, causing RF power to get reflected back into the radio.

Loose or poor shield grounding to the main board.

Add to the possibilities list that it's possible for the PTT
thing that sticks into the switch on the chassis to be too
long, if only by a hair. When recasing GP300s I've had
to shorten that gray plunger at times to get the PTT to
work properly.