so if my Vgs(th) max. was 4V, it would explain why a 3.3V TTL signal would be unreliable in keeping it on, correct? What it really wants is something more akin to 10V.

Yes and yes.
You could generate the 10V signal from a 10V-15V supply using an NPN transistor driver from the TTL signal (but that would invert the signal polarity).

 

Well I just so happen to have an n-channel IRL series right here in front of me...

@Zurn
I understand you solved this thread's issue, but your schematic doesn't show it, but.... you are using bypass capacitors on your MCUs, right? The primary purpose of a bypass capacitor with an MCU is to provide current quickly enough, in the event the power-supply cannot. This will help smooth your circuit's operation.

 

Yes and yes.
You could generate the 10V signal from a 10V-15V supply using an NPN transistor driver from the TTL signal (but that would invert the signal polarity).

No need, I just swapped the IRF for an IRL and it's working fine (so far).

@Zurn
I understand you solved this thread's issue, but your schematic doesn't show it, but.... you are using bypass capacitors on your MCUs, right? The primary purpose of a bypass capacitor with an MCU is to provide current quickly enough, in the event the power-supply cannot. This will help smooth your circuit's operation.

I do have bypass caps/dedicated DC switch mode regulators on the power supplies for my MCUs...is that what you mean? Apologies for the barebones schematic... it's just one small part of a larger system.

ANYWAYS... huge eureka moment. This logic level/non-logic level MOSFET mixup also solves another part of my system that has been tormenting me for MONTHS.

Thanks again everyone.