96 blazer runs good to it warms up

i have a 96 blazer runs good for the first 20 minutes.the it starts bucking,if you run it in 2nd gear it runs good.cleaned egr valve.no check engine light.

Welcome to AF.
Always check the fuel pressure on the road running after 20-30 minutes.
Specs 60/55. should hold around 62 -64 on the road.

How is the secandary ign and fuel fuel filter?
Good AC plugs ?
Good hot blue spark to all plugs.
How many miles on it and when was last good tune up.?

Thanks will check pressure mon.new fuel pump and tank,filter.new ac plugs.plus did intake gasket,and tuneup.its funny the colder it is the longer it runs good.is their 2 coolant sensors that maybe 1 is sending the wrong signal to the computer.Thanks

2 tempt sensors to VCM.
Air intake coolant sensor and engine temp to pc temp sensors.
Need a scanner to see what temp sensor they are sending into the computer.

But a run good after 20 minutes warmup is usually weak fuel pressure or a electrical part breaking down after hot.

Even a New fuel pump can be bad.
Was it a Airtex fuel pump?

Check the fuel pressure on the road running when it does it.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

Thanks will check