microcontroller - Odd NPN Circuit behavior

I've seen lots of useful threads here over the years but this is my first post.

I'm building a little box to switch an output based on RPM's watching the ground of an the primary ignition coil on an old car with points. I built an input circuit and it seems to work.... until the supply voltage gets too high.

Here's the whole thing:

I believe most of it is working ok - though I'm open to any and all suggestions - for completeness, my regulator is actually 9V not 8, but I don't think it is relevant.

My problem: The resistor (highlighted yellow in the simplified circuit below) seems to effect where my circuit works. My "function generator" spits out a 0-3.3V square wave and is input at "SIMULATED Tach-In". The real input isn't used here, but in the application this will look at the car's ignition coil.

The input to the NPN drops to ~1V if I keep turning up the supply voltage ("+12V"), and I don't know why. In fact, the output ("circuit output to CPU") stops switching after I reach a cut-off voltage which depends on the yellow resistor. See chart "Ohms/CutOff"

Does anyone have any idea why this is happening? I keep dropping the value, but I'm already using a 1/2W resistor and this seems overkill.

Please let me know if there's anything else I can provide to help with this.

THANKS! -Abe.

EDIT: More complete schematic added at (remove the space) abefm.smugmug. com/MegaSquirt-Stuff/Random-MS-Questions/i-sG85Vjc/A , not enough 'reputation' to post it here. Sorry for the remote link. The short version - there's a 470R resistor on my board, and a line which goes to the generator's CPU. (A "Jim-Stim" from the Megasquirt world, if you're familiar)

The good news is I got the pull up working on THAT board, so I now have a nice +12V signal which demonstrated my real input works fine.

Note: This was solved. For those interested in project, my next issue is discussed here: Mysterious Noise Issue in Automotive Ignition Coil Application, Arduino