First, make sure the problem is in the rig. I've seen poor connectors, coax and antenna traps cause this type of problem. You should know what your doing before attempting any radio troubleshooting.
Just to give you an idea, there are a number of relays in the radio. The following relays are before the transmit/receive (T/R) relay, RL5504 (Ant 1), RL5503 (Ant 2) and RL5502 (Ant 3). Next is RL5511 (T/R relay). Move your coax to antenna 2, switch to antenna 2 and see if the problem continues.
For receive, you can even bypass RL5511 (T/R relay), RL5501 (Protection Relay), and both RL1001/RL1002 (Attenuation Relays) by connecting a second antenna for receive only to the micro-tune port (From Micro-Tune J1003-1) and turning it on (Press AND hold MHZ/uT button by the CLAR/VFO B knob). You'll need a PL259 to phono-plug adapter. See page 29 of the manual TO MAKE SURE you plug into the correct plug. Do NOT plug the receive antenna into the 13.8 VDC or PTT plugs that are right below the micro-tune plugs.
After the micro-tune relay, there is RL1004 (amateur band-general band) and some band-pass filters.
Before attempting a fix, get a service manual. Don't work on this radio unless you have some experience.