Jim;
You have a truck which hasn't been maintained per mfg. spec..
The solution is to replace the vaporizer - regulator at approx. + 800.00 USD or take it to an authorized dealer to have it cleaned and rebuilt for a third of that. Once this is done and the regulator is reinstalled and readjusted. The engine will need to be relearned. At 200 hour intervals the tar drainning is required.