Muddasink,
From one newbie to another...I'm not sure of your vintage, but my 1974 has saddle tanks and I am having similar problems....just getting ready to get into more indepth investigation after 2 frustrating years.

What year is yours and I'll see if I find anything of note that might be worth passing on.

Mine seems to "stumble" not completely shut down..i'm guessing vacum leak somewhere along the line. Do you shift from portable to built in via 2 way switch valve or do you disconnect from one to the other??