On the trainer or outside?

Numb hands on the trainer is just a fact of life. Isn't as bad while riding outside but still happens. Relax your arms, bend at the elbows and change hand positions periodically. Still happens but can be dealt with. Using your core to hold yourself up instead of locked elbow arms helps a lot.

I've found the same thing switching from TT bike on the turbo to cross bike recently. I'd moved my bars up "for comfort and control" for a couple of cross races last year.
I've since dropped down to my old position and my wrists and lower back are better. I seem to have adapted my back to riding at a tight angle.

It happens about 8 miles into a ride and I am outside.

 
Not that anyone needs excuse to spend more money on cycling, but bars do make a difference. I went all in and put carbon Zipp SL-70 Aero bars on my current road bike. Comfortable as hell and I don't get the "cyclist's palsy" I used to get with round bars.

There are many things going on here.

First off, what part of your hands is going numb? The whole thing? Certain fingers? If so, which ones? Is it tingling? Both hands?

You "switched out the stem". From what to what?

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Travis Rassat
Vector Cycle Works
Noblesville, IN
BikeFit Instructor | FMS | F.I.S.T. | IBFI
Toughman Triathlon Series Ambassador

It starts from the palm of my hands and then it continues up my arms. Yes it is both hands going numb. I switched out a 110 stem to a 120 stem.

I have had this problem on several road bikes riding outside. Gloves and gel cork tape helps, but the biggest improvement I found was in saddle tilt. Simply put, when tilted nose-down to any degree, you are supporting more body weight and effectively pushing your weight backwards with your hands/wrists/arms. Getting my saddle tilt to 0 degrees/flat really helped to better distribute my weight and relieve my hands and wrists. I still get occasional numbness, but it typically takes several hours before this is an issue, and is easily shaked out. Most of your body weight should be on your sit bones. Once you get to that, you may have a bit of soreness for a short time there until your sit bones acclimate, but this is short lived.

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"It never gets easier, you just go faster."
-Greg LeMond

This sounds like a combination of too much weight on the hands with a poorly positioned hand on the handlebars/hoods which forces the wrist into an unnatural position. Contrary to what another poster stated, this is not just a part of life - it can be better.

Can you do the Steve Hogg point of balance test as explained here?

https://www.stevehoggbikefitting.com/...back-for-road-bikes/

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Travis Rassat
Vector Cycle Works
Noblesville, IN
BikeFit Instructor | FMS | F.I.S.T. | IBFI
Toughman Triathlon Series Ambassador

You are absolutely on the right track here.

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Travis Rassat
Vector Cycle Works
Noblesville, IN
BikeFit Instructor | FMS | F.I.S.T. | IBFI
Toughman Triathlon Series Ambassador

I've suffered the same problem on my road bike. Little and ring fingers numb by about 7k. Followed closely by the middle, then index. Started to become a safety issue for braking/shifting. I wrapped the bars in gel underpadding and lizard skins... made it too bulky, but helped the numbness. What helps the most is shifting my hands around on the bars so the pressure isn't concentrated in one spot for too long (i.e. alternate between drops and hoods). I just got done with a bike fit, so I'll be interested to see if that helps solve the issue. Picking up the bike tomorrow, and hoping for first outdoor ride of the season this weekend.

Don't lock your elbows straight. It cuts off circulation