1995 3.8L runing rough, stalling, shuttering

friend has a 1995 oldsmobile sihloutte 3.8L we got as a project to get running right, as 2 problems left.
1. When going down the road at any speed of 30+ MPH the car shutters whenever you let off the gas and it gets to 1500 RPMs, press gas and it goes away but comes back at 1500 again, it also shifts hard ever gear. we are planing on flushing the tranny and changing filter hoping that works, i think its something in the toruqe converter, but if you know anything it could be please suggest it...

2. When the car is idling at first it idles loppy between 500 and 1500 rpms, when you drive it and come up to a light, it idles really low (500+/-) and usually stalls, you start it and it starts right back up. its getting bad any ideas please help.

here is all we have done so far:
-changed plugs
-changed fuel filter
-changed PCV
-New ECU
-new ignition switch
-new exhaust

Ideas that could be bad:
-plug wires(look new)
-TPS
-CPS
-EGR valve
-O2 sensor
-fuel pressure regulator
-change tranny fluid

before i go to the local U-pull-it yard and get ever sensor I can find please let me know if you have any ideas...

Try the O2 sensor, if it's an older, high mileage van chances are it has never been changed. My 98 Van started stalling on right turns at part throttle, like a freeway onramp. Great, swinging onto the freeway, suddenly no brakes, no steering, no go with a crowd behind you accelerating and looking over their sholder for traffic.
I checked the O2 with my OBDII reader and found the O2 sensor had drifted so it read slightly rich at cruse. The sensor was telling the computer to lean out the injectors, so whenever something strange happened it would push the engine beyond the point where it could live.
Hopefully you don't have to go through what I did to change it on my 3.4L.
Good Luck
94

Hello! It may be the torque converter clutch solonoide hanging up.That would cause stalling when coming to a stop.That was the problem on my 97 olds..Diesel1962