My 1998 saab 900 turbo has been dying while driving. The issue only happens when it is very hot outside. After running the engine for 10-15 minutes my cars tachometer will jump up and down sporadically for a few seconds before hovering around 1000 rpm and finally dropping to 0 and killing the engine. The actual rpms don’t sound like they are fluctuating, just the tach. The car is able to start right back up and drive for a little before dying again. I just replaced: battery, crankshaft position sensor, fuel pump, fuel filter, spark plugs (Ngk saab specific), and transmission. If anyone can help me, you will be a lifesaver.