99 suburban, no power when hot

Friends 99 5.7 is in shop. It runs fine cold but when hot, lays down like fuel pressure is low. Has needed 60 at start, runs fine but when warm, does not. Pressure at manifold is around 50 to 53 or so at WOT but power falls off. Like an old car that has plugged filter or to small of fuel line but pressure does not seem to fall apart like it feels like it is. We have no codes and basics have been covered. It is at a good, honest shop that is not swapping parts and charging, they are trying to find real problem to fix it. Any ideas for things we have overlooked?
Thank you,
Tom

Welcome to AF.

When you say "basics have been covered" what does that mean as it could mean different stuff to us?

What's the scanner reading on the CTS (Coolant Temp Sensor) when hot?

Hi 77, the temp tracks with what it should be tracking. The TPS shows WOT and idle and there is no vacuum leaks or disconnected wires. We still suspect fuel pressure falling apart. Any idea what we should see at WOT , plus or minus 2 psi?
Tom