92 range rover backfire!!

[Chrisrover]

I have a 92 range rover classic with loss of power and bad backfire. All that I have done to it was replace o2 sensors ,cap ,rotor,plugs, wires, and fuel pump.but nothing helped!!. Computer gives out a code 69 gear switch which was replaced by dealer ,but code comes back check engine resets when turning car off. And Flickers over speed bumps and rpm drop too. Any help would be appreciated thank you.!!