sorry, assumed you had done that already, heat and lp quality here make us check the lp system first.
We have resolved the idle issue by adjusting the idle mixture at the propane regulator. You can do this by knocking out the cap covering the mixture screw on top the regulator & turning till the surging stops. Why Toyota puts a cap on the regulator to cover up the screw up is beyond me. We have had issue's with the Aisan regulators used in the 7 & 8 series Toyota's but never new to knock out this cap or that it was even possible. Only after looking into our problem, but in other model Toyota's on Forklift Action having the same issue did my Technician try this & it worked. Thanks to all involved & using Forklift Action, Paul.
My Service Technician cleaned the throttle body with no change to the idling issue.