Runs fine at less than half throttle but give it half throttle or more it wants to die. Whats up?

 

I hope you will post the cause when you find it. I have this same engine and I would like to know where to look if mine ever does this.
I could not find your exact symptom in the Chevy manual but I'm posting the nearest thing listed. It's a starting point. In any case, all
internal combustion engines operate on the SUCK - BANG - BLOW principle. Suck in air and fuel, ignite the air and fuel and blow the
burned air and fuel out the exhaust pipe. All three are addressed in the diagnostic below. If this were my car I would start with the
exhaust. My friend owns a muffler shop and people come in with vehicles that won't go over 45 MPH because the catalytic converter
has clogged up. Your LE8 engine has three. You can do a quick check with an infrared thermometer. If the inlet and outlet pipes
are the same temperature that cat is probably not working If one cat is much hotter than the others it is probably plugged. Not saying
that is your problem but that is where I would look first.

 

Actually its not mine. Its for sale at a reasonable price and the listing says that's whats wrong with it. I thought if I could figure out the problem I could get it for a better price. Then fix it. Thanks