Hi Greg;
Having expierenced the issue from the time this series and model was introduced in 1997 to the last production in 2004. The timing you have set is correct for the fuel supply and engine family. Do you have the fan belt deflection set at 0.43 to 0.51? Are the plugs being used BPR4ES and gapped at 0.035? Is the distributor's air gap at 0.018? Is the engine's max full load speed at 2300-2600 RPM and the no load speed at 2800-3000 RPM? Is the full load fast idle control adjusted correctly and is it functioning correctly? Are the LPG components OEM UL rated as per factory bulletin of Apr 24,1998? With the air cleaner filter installed has the power adjustment been made properly? With Impco system is the secondary spring correct and is the air valve pressure for idle and full speed correct? Has the engine been checked for exhaust gases in the coolant?