HELP! Truck Idles Low and then Stalls after Warm-Up
 

I just installed new stainless headers, 3" exhaust, high flow cat, flowmaster super 40, and o2 sensors. While I was doing the headers, I replaced both front fenders because they were rusted. I also did some welding because a few of the header bolts broke.
Now, after I got the truck all back together, the truck will start up and idle real low(300-500rpm) and when the truck gets to operating temperature, it stalls.
At first I thought it was a heat issue, so I wrapped the headers, fuel lines, and starter. Didn't help.
The check engine light was also on (P1687 - No MIC Bus Message Received and P1391 Intermittent Loss of CMP or CKP)
So I checked all the bus wires and camshaft and crankshaft position sensors, and they checked out fine. The check engine light also went out after checking everything.
I took it for a test drive and the ABS light and Brake light came on and the speedometer was stuck at zero until I got to 20-30MPH. And then it eventually stalled, unless I kept on giving it gas. So I replaced the Vehicle Speed Sensor and ABS Speed Sensor. Didn't help either. Speedometer is still out of calibration.
Other things I also tried was replacing the Idle Air Control Valve and cleaning the throttle body.
I also thought that since I did some welding, I might of fried the MIC Computer (Instrument Gauge Cluster), so I swapped it with a junk yard one, and it still stalled.
Battery is reading 12.15 volts and 13.8 volts when running.

Also, my uncles scanner tool can not access data on the truck, like it used to before this all happened.

I am tired of putting money into replacing parts. Any help will be appreciated.

Welcome to DF ;) Please do not post tech questions in this section(FAQ/DIY) . Post tech questions in the tech section. Thanks :) I'll move this to tech for you.

Thanks zman, still getting to know the forums...

What did you have to weld?

Get battery tested. I was having a similar problem, turned out to be a bad battery.

I Had To Weld Nuts On The Ends Of The Broken Bolts To Get Them Out. (Sorry, My Phone Is Capitalizing Each Word)

I'm thinking crank sensor. A bad one does just what you are saying, stalls when warmed up. I see you replaced o2 sensor. And what brand parts are you buying to replace the sensors ? And did you disconnect the negative battery cable when doing so?

Not the battery, I tried a battery from my car and it still did the same thing.

Crank sensor is the only one that I haven't tested yet because I can't locate it. I have the book on the truck but still have not located it.
But why would all of a sudden, it goes bad? I'm wondering if it is just the wiring has a short or something, but it will take forever to find it.

The o2 sensors I got are Bosch brand. The idle air control valve, speed sensor, and abs speed sensor are all Duralast brand. and yes I disconnected the negative terminal when I installed them.

Bosch and duralast are exactly the brands you DONT want to use. ;) Anyway, the crank sensor is behind the passenger side head sticking out of tranny bell housing. Very hard to see and replace. But they are very prone to going bad. Get one from dealer or a Standard Motor Products brand.