[Novablue]

First thing that came to mind was spark plugs. First thing I'd check would be a vacuum leak.

Did you do the conversion to LPG or did you have it done? I'd love to look at that set up. Most gas vehicles are not really built to handle LPG for extended periods so I'm guessing some mods came into play. If not, well you might be experiencing those types of problems now.

Do you have any codes (even if the light isn't on you might still have a code stored)? PVC, vacuum leak, sensors, lots of things really could come into play.

 

[Novablue]

Cool to hear!

If all else fails you could have the ECU flashed for a higher rpm, but I suspect that would be an expensive and temporary fix.

I'm always fascinated by LPG conversions, here in the US it isn't really cost effective to do it but there is a small group of people that like to tinker and make their own as a hobby. When I was seventeen I was tasked with getting a dodge truck to convert to LPG. It was a fun project.

If you can't figure it out you might want to run by that shop. Unfortunately they maybe the only ones that can help you. Hopefully someone else can way in too.

 

[Novablue]

My preferred method is a smoke tester, but seeing as you don't have one (who does? well cardoc might )

How To Find A Vacuum Leak - EricTheCarGuy - YouTube

Sorry about the annoying video...

Ironically LPG is one method which is why I included that video (I know you might have access to LPG). Please don't use carb cleaner like this guy does. If you must make sure you have a friend [That you trust] with the fire extinguisher at the ready. Too many things in old cars can ignite that stuff, even subies.

I need to learn how to embed video...