loosing power
are you saying the car is falling on its face in 3rd gear and it was not before?
or are you saying when it shifts into 3rd it feels like the rpms are goin up but the cars acceleration is slowing?...
the trouble being gear dependant, points to the tranny itself, most likely your 3-4 clutch bands. time for a freshen up.

(hope it turns out to be something cheaper,good luck)
hey look at it this way... taking it to a normal tranny shop for an overhaul would cost you as much(normally) as it would for you to step up to a nice built tranny.... so now is a great time to get that upgrade out the way....
(pm sent on good tranny builder)
1000 miles over due on a oil change(while not a good thing to make practice) will not decrease your power much ...(if any)
a ported TB, while it may help you pick up 3-7 peak hp will not create a seat of the pants differance.
the reason your friends grand am falls on its face when it shifts into 3rd, is because (while it is a fun little car to drive) a grand am does not have **** for power(yes the gt included) in first and second gear, the fact that the gears are so low make it feel quick off the line... however, in most 4 speed auto trannys 3rd is your 1 to 1 gear meaning in 3rd your motor has to pull the car without the aid of undergearing. being that the GA GT doesn't have alot of power to begin with, it feels like a significant drop in power shifting into 3rd....(my 2000 GA GT did the same thing)
the 4th gen fbody , does not have that problem...
the problem it DOES have in an auto tranny that, in most cases, is not strong enough to handle the stock power of the car, let alone when you start modding it.
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looking at your post it appears that he had his TB installed right before it started giving him problems... is that right?
if so, then someone either leaked coolent, or put there greasy hand on the IAC(idle air controler), to fix, pull the TB back off, right under it is the IAC, clean it with isopropenol...

you just proved my point there chief, all of the cars you just listed there are EXTREMELY slow... not a single one better then a 15 second 1/4 mile time.
my 13.4 lt1 is SLOW... my 11 SEC ls1 is not fast on this site... if you own a 4th gen v8 you cant honestly believe what you just wrote...
can you?
...im as dumbfounded as you as of to why both me and my sis' cars have run like $h!t, then ran fine after an oil change...
This doesn't apply to you...Could replace plugs and wires, that also could shorten u power.
Good Luck

