850, S70, V70, C70 Engine does not start, failed start

[reise]

Hello people. For about 3 years I have had a problem with my C70 (1998) not starting the engine. Starter motor turns, but the car does not start. Sometimes for several months the problem does not appear, while in some periods it almost always does (both in winter and in summer). Then when I manage to start it, the car is perfect. I recently replaced (original Volvo purchases): air mass meter (I had very high consumption and irregular engine) and accelerator position sensor (gear changes were out of order and the car jerked), both problems solved. While years ago I bought new ones: distributor cap, distributor rotor (Facet brand), spark plug cables (Tesla Made in Czech brand) and spark plugs (original Volvo for C70 Turbo). While they are still original, never replaced: starter motor and coil. My 1998 C70 is very similar to the Volvo 850 95/96/97, as it still has only 1 ignition coil, cable throttle, no phase shifter. What can I check to fix the problem? I want to underline that if I manage to get it started, the car is perfect with the engine.