hmmm, if it runs perfect on LPG and then on petrol has the pop pop issue could be a valve issue.
The way LPG combusts can hide issues with valves in that it runs perfectly smooth on lpg but not so on petrol
Id be having a really good look for vacuum leaks as well.
is it injected LPG or...
is the pop pop pop in the exhaust at idle and felt like a pulse when you put your hand near the tailpipe? An incorrectly adjusted exhaust valve or burnt exhaust valve can exhibit similar symptoms as well as unmetered air between the MAF & throttle body.
with #5 plug try swapping another lead...