I'm with the fuel pressure, my 350 isn't multi-port...TBI, but the fuel pressure regulator spring rusted and broke giving a loss in pressure and the engine responce sucked until I replaced the spring. Not sure how it's regulated in the LT1

 

Is it new enough to throw SES codes? Off hand it sounds a little like a vacuum leak to me also. My Ranger had a similar problem, in 4th and 5th gear it would bog down and the engine would "hiccup". The code it threw was something like "bank 2 running lean". I never saw a change in rpms though so I tried replacing other parts that did nothing. It turned out to be a gasket under the intake plenum that wouldn't let it pull a vacuum.

 

Check all the vacuum lines. Tiny and BIG ones.


My old boss had a simular problem with his F-150. There was a big hose at the back of the engine and while cold the crack was closed, but when the engine warmed up and so did the hose. The crack would open allowing air into the intake.

 

Try this first.Take the battery loose and then remove your mass air meter.Clean the inside (little fine wires in the middle) with contact cleaner(electric parts cleaner or brake cleaner)..The mass air units are known for getting dirty and effect the drivabilty of these cars.

If not check fuel pressure if its dropping or you dont have the proper volume the pump is going out.

 

DONT TOUCH THE ELEMENT/S IN THE MASS! Just spray it down. Then let it fully dry.

 

Forgot about that..But you shouldnt be able to touch it if the screens are in place.

 

Not all mass's have screens.