Is this an internal tranny problem?? (440-T4)

It's been OVER a year now and my 89 Cutlass STILL has a harsh/bangy/neck-snapping 1-2 Upshift!
I have replaced all front-end components such as wheel bearings/hubs,
ball joints, tie rod ends, AND both complete axles/drive shafts. I also replaced all engine mounts including torque struts. All of this has not cured or helped the 1-2 Upshift in the slightest bit.
So now I think it HAS to be internal.
I've had the transmission serviced twice since it started. I had the Vacuum Modulator replaced. Again, none of it made a difference. The only thing I've never touched is the TV(Throttle Valve) Cable but this supposedly only helps control the timing of the shifts and has nothing to do with main line pressure or shift "feel", which is controlled by the VM.
There's only a few things that make me wonder if it really is internal. First of all, this is the 3rd transmission my car has had and all 3 of them have done this exact same thing. And two, it's quite intermittent and really only seems to do it once the fluid is hot. The hotter the engine gets, the worse it shifts. But then there's that intermittent thing where for some reason it might do it once even when it's cold, and might NOT do it even when it's hot...
When the Vacuum Modulator was replaced, they replaced it with an adjustable one. I've been to timid to try and adjust it because I've heard people say that a transmission that shifts too "soft" is far worse then one that shifts too "hard". So I don't want to turn the VM's screw and at the same time lower main line pressure and cause my transmission to slip on upshifts.
I've used my Vacuum Pump to apply pressure to the modulators vacuum supply line to check and make sure that it held and it did without any problems.
The 2-3, and 3-4 upshift are a-ok. I 4-3, and 3-2 downshifts are ok. HOWEVER, sometimes the 2-1 downshift is a little "clunky". Especially if I am in 2nd gear, and force it to downshift to 1st by using the gear selector. Another thing that it often does when hot is "clunks" into Reverse gear. But again, this is intermittent.
So I'm guessing that it's time to give up and just get it re-built but I don't want to waste money like I already have done...especially with the axles.
The first thing that comes into most mechanics minds when the drive it and feel the 1-2 shift is the Vacuum Modulator. But I tell them it was replaced and so they move on. But I wonder if I did somehow get a defective one? Or perhaps the rubber connectors that are used on the modulators steel supply line are somehow leaking when they get hot??
It's hard to describe the actual 1-2 shift...but it kind of feels like for a split second, the transmission goes into neutral, and then "bangs" into 2nd gear. This forces the car, and the passengers heads, to "jerk" forward. Obviously the higher the RPMs when shifting the worse it is. If I accelerate super slow, it's almost un-noticable.
What do you all think?
THANKS!:)
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On 18 May 2004 12:11:39 -0700, snipped-for-privacy@yahoo.ca (bigbossfan80) wrote:

Something internal and pressure related..
If I remember, the 1-2 shift commands a much higher pressure than say the 2-3 shift. Maybe something sticking in the valve body? Slipping clutch pack?
I think I'd start with pressures..
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