I have 01 MC 3.4L with approx. 175000 miles on it.
Sometimes it stalls out and dies but runs fine when
first started when cold, but after about 5 or 8 miles it
looses power and will only spit and sputter along bout
3 mph. It idles just fine. What possibly is wrong? I
would appreciate any input you have. Thanks.

 

If I had to make a guess from your description I would think one of 3 things I have listed them in the order of the least cost fix.
*Could be something as simple as the Air Filter
*You could have a Fuel Filter dirty & plugged issue.
**But it is most likely the Catalectic converter that is plugged this has happened to me on a few cars. Everything runs good but the get up and go is gone.

 

I would guess the catalystic convertor is plugged. Easy way to check is have someone give the engine gas and check if there's a lot of air flow coming out of the muffler.

 

Also check the fuel pressure when cold and when hot to see if possibly the fuel pump is on the way out.

 

One of my questions is how old are the spark plugs and wires. If they are still factory, you have exceeded the life expectancy of them. I would do that maintenance before going crazy elsewhere. Factory spark plugs are meant to be changed at 100K miles, with reduced fuel economy, life can go 150K, beyond that, you are on borrowed time.

Do NOT use Bosch plugs (wires are fine, plugs are not). Stick with AC Delco or NGK plugs and get platinum plugs. Rockauto.com is a great source for these parts.