Need advice on sick GXP. SES / Vacuum Leak?

[BlackGTP01]

So I have an 01 GTP also right now that's down and out. This post is about my 06 GXP DD
2 Months ago it started throwing the Service Engine Soon light, I bought a scanner and found it was the usually Gas Cap Code. Bought a new cap, deleted the code for it to come back a few days later.
Next I put in a new EVAP Canister, deleted the code, it came back.
The car has a constant gas vapor scent but has never leaked a drop.
Last month when I replaced the Canister I dropped the filler neck and inspected it, wrapped it in electrical tape, hoping there was a pin hole somewhere, still had no luck.

On my way back from San Fran to Reno with a car full of people the tranny started acting funny. Down shifting when it didn't need to. My friend said it might be a vacuum leak causing everything. Just ordered a brake bleeder and vacuum pump kit, should be here tomorrow or Thursday.
This morning I left the gas station and it slammed into 2nd and started burping and sputtering when it wasn't even shifting. Traction control and Stability both turn off on their own.

I put it in Manual and as long as it wasn't shifting it was acting fine again.
I know these cars transmissions aren't the best but i'm clearly hoping that isn't the case. Only has 72,xxx on her.

Any one has any input or experience it would be greatly appreciated!

 

[Scottydoggs]

you need a smoke test for the evap, lines can rust out and you smell gas. my vent tube tank to filler next rusted a part, smell of gas was strong.

could be a bad maf causing the poor shifting. other then that check your trans fluid color and smell. scan it after it acts up dont shut the engine off, scan it running still. see if you get a pending code for the max adapt shift code. if you got that a shift kit will band aid it.

 

[BlackGTP01]

you need a smoke test for the evap, lines can rust out and you smell gas. my vent tube tank to filler next rusted a part, smell of gas was strong.

could be a bad maf causing the poor shifting. other then that check your trans fluid color and smell. scan it after it acts up dont shut the engine off, scan it running still. see if you get a pending code for the max adapt shift code. if you got that a shift kit will band aid it.

I pulled the tank filler off and there was no corrosion or pin holes, which I was hoping to find. The car has always been on the west coast so there is virtually no rust.

I parked her until the weekend, i'll check all of the things you suggested and repost hopefully when I get a solution.
Thanks for your input

 

[BlackGTP01]

So I've been messing around with the car with no luck and it's getting worse. This is my daily driver and it's to the point where it's acting up to the point where I'm ready expecting it to break down. It slams into gear and even in overdrive's attempts to downshift when it shouldn't be. Other times it runs fine.
So I just scanned her and I have a laundry list of codes.
C0240 - traction control
U0073 - control module communication bus off
P0700 - transmission control system
U0001 - high speed controller
C0898 - no DTC

The car is an 06 with 73000 miles on it. I continue to have problems with this car lately. Any input or suggestions is appreciated

 

[Fivefingerdeathpunch]

Find a scanner that can read the transmission codes itself.

The P0700 code means something is wrong with the transmission.

 

[blueguy]

I had a couple of those when the TQ Converter let go.

Sounds like the converter took a crap from high temps.

Happened to me at 60k miles. Join the club. Mine would randomly shift or feel like it was in neutral; then slam HARD into gear...sometimes would even chug like a huge vacuum leak or misfire/MAF issue. ALL was transmission related. Time for a rebuild, man. Put a transmission cooler on it after the rebuild and bypass the radiator and get it tuned when you do. Worth it's weight in gold.