4506
If you are familiar with the internals, try a complete dissassembly, thorough cleaning, and light re-lubrication first.
While you have the gun torn down, check the rear of the drawbar and the body of the disconnector for damage, and the disconnector/drawbar for free movement up and down.....look for evidence of contact between the inside surface of the drawbar and magazine(s). (May show up as small shiny spots or drag marks on the magazine body) Check the integrity of the trigger plunger and spring....this holds the drawbar in proper position. Check the levers for damage or burrs.
Check the hammer and sear for evidence of damage at contact points....bevel/angle on sear and SA step on hammer. Are you using OEM magazines?
Others on the forum will come along with additional ideas.