Try cutting the bias pots to minimum bias.? Then turn one of them up to the desired bias.? Then turn it back to minimum.? Then try turning the other pot to the desired bias.? If each pot will adjust the bias to the desired amount and even a lot more, then the IFR510 connected to that pot should be ok.? If you can not raise the bias to both of the equally I would look at changing the IFR510.
I'm getting the same 5W output on CW and SSB (with a loud "helloooo").? I also noticed that my PA bias pots when set to the proper 100ma per IRF510 are at slightly different settings.? RV1 is turned all the way up.
Does this sound like a drive level issue?? Or do I maybe need to replace the IRF510's?