Good to hear that progress is being made!
What is the FSCTRL1 (IF frequency) when you use 650 and 812 kHz RX filter BW? Try with FSCTRL1 = 0x0E.
Also, the AGCCTRL2, 1,0 settings are important. What are these settings for the cases where you have high packet loss? Try AGCCTRL2, 1,0 = 0xC7 00 B0 (if not already done)