Small problem with the starts hot engine! - Volvo 740, B230F

I have a small problem with the starts engine when is hot.
When it is cold - starts without any problem, and when is hot, and I will
switch off him and in a moment I will start, then also it starts very well
!!

But when the hot engine I will turn off and I want to start him after 15, 20
minutes, I have with this a small problem. The engine will starts on the
first time, but I must several seconds use the starter to turn on the
engine!! I use benzines and LPG Gas and the problem is in both fuels (so I
think, that it's not a problem with the injection or fuel pomp, because on
LPG Gas the injection and fule pomp is turn off), so I suspect the ignition!

I have an engine B230F LH2.2, the year 1988. Perhaps the Hall sensor is
damaged? How I can test them? Some symptoms are indicator for test the Hall
sensor??

To ask for the advice...

all the best...