The car is Manual 2000 Skoda Fabia 1.4 with non-stock LPG system installed.
So the situation.
I'm driving down the slight hill in 3rd gear.
The engine hums at 2k rpm.
Then when I'm near the end of the hill, I start applying brakes, the RPM goes down to 1,000, I switch the gear to neutral while still applying the brake, and the fun begins.
The moment I switch to neutral, RPM drops to 500, rises to 800, drops to 500, and rises to 800. And If left like that, the engine will just stall.
Sometimes flooring the gas pedal for a second fixes the problem, and sometimes after it just returns to the 500-800-500-800 cycle right after.
But, if I stop, turn the ignition off, and turn the ignition on, the car Idles at 800ish RPMs like it's supposed to.
THE MESSED-UP PART is that I can't reliably catch it. Sometimes it happens, sometimes it doesn’t. Sometimes, when I enter a turn, I press the brakes, press the clutch, more brakes, turn the wheel, and then try to accelerate - only to find that the engine has stalled. No jitters, nothing. Just the red battery indicator hinting to me that the engine is off.
My car mechanic said it happens only on LPG, but then when I caught it again, I tried switching to gasoline, and it changed nothing. Same old swing from 500 to 800. So I guess he has no Idea what it could be