I've had this hair puller issue before- even lost a customer over it. If you have coolant flowing thru the vaporizer as soon as it's started, coolant flow isn't the issue.

Are you sure you have the correct regulator installed? (the correct spring pressure (blue or orange I think) for the size of the engine)?
I have been having all sorts of issues with the newer Model J's recently- have been converting to the Cobra's as I service them.

Last but not least- is this a new customer for you? I ask because the latent water content of the propane could also be an issue. The "commercial" grades of LP tend to freeze up faster than HD-5. This is where I ran into trouble & lost the account- the LP itself was to blame for the freezing & I found that out toooooooo late!