We have seen hundreds of GM300's in all bands with a wide variety of faults, including freq drift, int/no rx, howling feedback rx, rumbling unintelligible tx, etc. most as a result of J6, the header that you said you've cleaned. For a while, our bench tech had been replacing J6 altogether, including the female ends on both the logic and rf board. However, a couple of yrs. ago we just started cleaning the header with a fiberglass burnishing tool, and coating it lightly with non-drying electrical joint compound (same stuff the power guys use on transformer posts etc.) and spraying out the female connectors. This has worked on 99% of cases, and I have never had a repeat offender after this treatment. However as the above posts indicate, there could be other issues.
My 2c -RDO2