Hi Kat,
thanks for your schematic.
Your problem is two transistors Q8 and Q11 are going to saturation.
This makes the pulses longer.
Are you sure with D1 and Q6 ?
I don't know the voltage of the black level, but it seems possible
to feed the sync from pin8 via a resistor to emitter of Q7.
In this case you don't need R23 R28 R29 Q8 Q11 and R17 is 100 Ohms.
(higher bandwidth!)
For serration pulses a solution with fH x 10 and a modulo counter is
independent from the standart. But the fH x 2 and monostables works fine
too.
Kind regards
Darius
Reason for editing: I didn't see the minus 5V, I thaught it is ground
this negative voltage rail makes it complicated . Speedup your transistors Q8 and Q11
with a schottky speedup Diode BAT46.