I acquired a hand me down Titan X Maxwell(EVGA SC model) from my son.   Been serving him well for years now.   Still runs fine but the idle temps seem kinda high to me.   I can boot up the machine and as soon as it gets booted up I pull up a temp monitoring app and the temps are already in the low 50s.   After several minutes it hovers around mid 60s.  If I run a gpu intensive app like Unigine Heaven benchmark the temps go up to 83/84 and sit there which I think is about normal for this card.  Using no tweaking software like MSI Afterburner or EVGA Precision X1.
I opened the card and cleaned it out good.  New thermal compound on the gpu, etc.   Doesn't seems to make much difference.   I'm scratching my head trying to figure why it's running so hot at idle.
 
It's in a good air flow case.(Corsair 760T)    At idle fan speed shows 28%/1320rpm @67c

I would suggest checking the thermal paste if you don’t mind taking the cooler off.

Don't mind at all.  And actually I've already applied new thermal compound twice.   The first time I opened it up partially, not removing all the fan shrouds, to access the heat sink.   I applied some NT-H1 compound.   After cleaning the old dried up compound obviously.   I did get a small drop in idle temps but not that much.   I think when I got the card and checked it initially it was hovering in the low 70's.   That is when I decided to take it all down, fan shrouds and all.   It really wasn't all that dirty inside but cleaned the cooling fins and more so the fan itself.   I then applied some Kryonaut Thermal Grizzly to the gpu and spread it out well.   This is where it's currently at.
 
So not sure checking the compound again is the answer.

Have you verified that nothing has clogged the fins above the vapor chamber? If some dust got in there, it could definitely cause issues.

If you have verified, and you have redone the paste, then it may be a different issue completely.

This may be a silly question, but have you looked at the fan while it was on to make sure the fan is actively spinning?

Yep, fins of the vapor chamber are completely clean.   And yes one of the first things I did was check to ensure the fan was spinning.  It needed cleaning more than anything else.

I had two of these & they can run warm ... why I used water blocks & custom loop
 EVGA - Product Specs - EVGA GeForce GTX TITAN X SC GAMING
 
First question - Did your son put in a Modified BIOS ?  ... may people did, to get better performance from these cards .. the heat jumps with the voltage
 
Second - when changing TIM the second time, did you have "Full coverage on your GPU die" - "good pattern" with no voids or missed spots (bare die)

If you have checked everything else, to include what CoolGTX mentioned above, you also may be experiencing issues with the vapor chamber itself.

Cool GTX
I had two of these & they can run warm ... why I used water blocks & custom loop
 EVGA - Product Specs - EVGA GeForce GTX TITAN X SC GAMING
 
First question - Did your son put in a Modified BIOS ?  ... may people did, to get better performance from these cards .. the heat jumps with the voltage
 
Second - when changing TIM the second time, did you have "Full coverage on your GPU die" - "good pattern" with no voids or missed spots (bare die)

I am my son's tech support!   ;)   The only hardware mods to his equipment are done by me.  So far the only firmware update was the Nvidia one.  And yep used the paddle to spread the TIM evenly across the die.  No voids.   One thing I had thought about attempting is to use some thermal pads on the metal frame that surrounds the GPU itself.  If you know what I'm talking about.  It might not matter but I was wondering if causing contact between the metal piece and the heat exchanger might help or not.  I have some 1 mm pad material but looks like it would take something thinner....like .5 mm.  I'm honestly not sure the purpose of that metal frame but it looks like there is a slight gap between that and the heat exchanger.

"frame" protects the Die ... not a place to add pads - you want good contact between the cooler & die - from the GPU cooler mounting springs

I will suggest this card may just be running warmer naturally.  I have never looked at the temps the whole time my son was running it so can't really compare how it ran when new compared to how it's running, temp wise, now.  Doing a Google search I can see where others are seeing idle temps in the 30's.   That just makes me scratch my head.  I might try bumping the fan curve up via software some to see how that helps.