so this has been driving me nuts for about three weeks now. my IC-7300 will just randomly lose a ton of receive sensitivity on 40m, sometimes 80m too but mostly 40. it'll be fine for an hour, then the noise floor just drops way down and signals that were S7 are barely tickling the meter. if i go to 20m or 15m it seems totally fine which makes me think its not a preamp issue but what do i know.

first thing i did was swap coax thinking maybe a connector was arcing or something, no change. checked the antenna switch, reseated all the cables inside the shack, even ran a temporary feedline directly to the radio bypassing my usual setup. still happens. the weird part is it usually comes back on its own after maybe 20-30 minutes, sometimes just cycling the band and coming back fixes it for a while.

i did a factory reset last week hoping it was some weird software thing, no help. the radio is about 4 years old and has maybe 400 hours on it, not a ton. has anyone seen this behavior before or have any idea where i should even start looking? i dont really want to ship it to icom unless i have to.

that intermittent-on-one-band behavior is suspicious for a band switching relay issue. the 7300 routes different bands through different relays for the BPF stages and if one of those relays has a dirty or worn contact it'll do exactly what you're describing — works fine when the contact happens to seat right, then degrades when it shifts a little or heats up. the thermal aspect you mentioned, coming back after 20-30 min, could be the relay contact expanding slightly which either makes or breaks whatever marginal connection is there depending on which direction its failing.

not a trivial fix to do yourself since you'd have to get into the RF board but if you're comfortable with SMD work it's worth pulling the top cover and looking for the BPF relay bank. some guys have had luck just reflowing the solder joints on those relays. there's a service manual floating around if you search for it, has the relay locations marked. if you're not comfortable going in there honestly icom's service isn't that expensive and they'll find it fast since they see this stuff all the time.

had something similar on my 746 pro years ago, turned out to be a cold solder joint on one of the band pass filter switching transistors, not even a relay. took forever to find because it was so intermittent. the way i finally nailed it down was with a heat gun on low just kind of waving it around different sections of the board while watching the S meter — obviously be careful doing that and dont let it get too hot anywhere. when i hit the right spot the signal would drop immediately and come back when it cooled. let me localize it enough to find the joint under magnification.

might also just be worth checking if icom has any service bulletins for that radio, i think there were a couple issued for the 7300 in the first few production runs. cant remember what they covered though.