hi,
So its not really a SG, but an interrupted IR beam circuit.
On your scope when using the IR detector, what signal to the tacho display, do you measure.?
I need to know the amplitude of the signal and is there any 'dc' offset.
If there is a 'dc' offset this may be cause of the failure after a few seconds.
The idea of using a coupling capacitor would be to remove this 'dc' level.
Post a picture of the scope display, but tell me the scope settings, mV/cm, ac or dc etc... OK.
Do you have a diagram of you final IR detector circuit that you could post.?