Hi, sorry what is a PN ?
Texas is : SN74HCT74N
NXP : 74HCT74, i have to look.
Just take a look at the datasheets. The NXP one has a schmitt trigger in the clock input (actually most NXP logic parts have schmitt trigger inputs AFAIK), wheras the TI most likely doesn't (the DS doesn't make any mention of it).
NXP:
https://assets.nexperia.com/documents/data-sheet/74HC_HCT74.pdfTI:
http://www.ti.com/lit/ds/symlink/sn74hct74.pdfThe lack of a schmitt trigger in the TI part makes the internal clock signal bounce in your case (it's not a digital signal with "clean" edges but an high-pass filtered signal.)
Don't remove the series resistor. You're going to kill the 74HCT74 eventually. If you didn't kill it yet, it's thanks to the high-pass filter which limits the duration of the 12V pulse and the internal clamping diodes. You may end up frying one.
With this kind of circuit, your best bet is to use a 74HCT74 with a schmitt trigger clock input such as the NXP one (I'm sure there are other vendors with this).
You could also add a schmitt trigger buffer in front of it, such as the SN74LVC1G17 (it's a single schmitt trigger buffer, the LVC series should be ok for your requirements and it's easily available whereas the 74HCT1G17 is not.)
If this was meant to be in a product, the second option would probably be better, because using a "standard" part (a reference available from many vendors) and requiring specific features in it from a specific vendor is a recipe for potential supply disaster later on.