Most likely cause is restricted fuel flow. As one of the other poster said, check the fuel flow as close to the injector pump as feasible. You should have a continuous full delivery of fuel. If the flow dribbles or is intermittent, you have a restriction somewhere between that point and the tank. The usual culprits are the tank outlet strainer, pinched or damaged fuel lines, crud in the filter head, clogged fuel filter(s), clogged screen in the lift pump (if you have one) bad lift pump, etc.BX23s starts and idles normally 5-10 minutes ok, loss of power moving will idle up then down 816 hours changed both fuel filters cleared fuel tank vent line, bled air at inj pump still the same condition.