Hard to start when cold.

My 99 Suburban is hard to start after setting out all night in the cold. Had to use either this morning. (temp 28 degrees)
Starts ok after warmed up.

One of the leading causes of hard to start cold is low fuel pressure.
But a good tune up may also be needed or help.
What engine and How many miles on tune up?

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.

MT

It's the 5.7 ltr.
I'll check fuel pressure tonight. I replaced the fuel pump about a 16 months ago. I noticed the fuel pump pumps up when you first turn the key on. The impeller on the pump may be getting worn.

It is normal to have a two second prime up when key is first turned on.
Pumps are roller type. No impellers.
Also have you checked the fuel filter?
Remember you need 65 lbs of fuel pressure on a cold start to squirt injectors.

Even a few lbs low can give a slow or not start.

Go find a Carquest Auto parts store for a fuel pump,they switched to Delphi and ditched Airtex for supplier for their fuel pumps which was a smart idea.It even has Delphi's logal on the box.Carquest did have problems with their fuel pumps when Airtex was their supplier.Airtex would not cover them under warranty calling it installer error.I know one owner of a Carquest auto parts store that went through this.

Checked fuel pressures. Ignition on - 55psi, Running - 60 psi.

What you have posted is low.
But you need to go threw the hole test and post back the full result to get to the problem.
MT