05 xc90 t6 transmission issue
 

My fiancée fell in love with an 05 xc90 t6 and she wanted it, and the person selling it agreed to buy new struts for it, new inner and outer tie rods and new upper and lower ball joints. All we had to do was buy a new set of tires.
And we found a killer deal on some new bridgestone tires that only had 5000 miles on them for 300 dollars.
The previous owner also provided us with the history of the car, and my main concern with this car was the transmission, but the previous owner showe me the warranty paper work showing the transmission had been rebuilt by volvo at 105 thousand miles, and both of the cv shafts were replaced at the same time as the transmission and both rear wheel bearings had been replaced at 80 thousand miles. We bought it with 147 thousand.
Since all this work had been done to it and the previous owner bought all the new parts I decided it was worth rolling the dice on, specially for the price they were selling it at.
We have had it for 3 months and we have only put 3500 miles on it.
The first week we got it I changed the oil with synthetic 5w-30 and new oil filter and air filter, I also replaced the spark plugs, and I serviced the transmission with a new filter and 6 quarts of dexron vi and a bottle of lube guard red.
This last Friday night my fiancée gets in it to come home from work and she said she noticed it was driving funny in 1st gear, she said it wasn't taking off like it normally does, and half way home the service transmission required light came on, when she got home she told me about it and I went out and got in it and I noticed right away it was slipping pretty bad in 1st gear but the transmission service required light had went away when she shut the car off.
It drove fine in all the other gears except for 1st gear. I checked the fluid and it is full and it is still bright red and it still smells normal..... This is where I am confused, in my expierence when a transmission is on its way out the fluid will be burnt and stink. And also in my expierence the transmission will give you some kind of warning or hint that it's on its way out, we have never felt any kind of slipping or any strange behavior from it at all.
Today I went out and started it up and I took it for a little drive around my neighborhood and it wasn't slipping today and the light didn't come on.
So what should I do? Should I try dropping the pan and servicing it again and cross my fingers and hope it goes for while longer? I'm not scared of removing the transmission and re-building it myself if I have too, and if it does burn up that will be my only option because I don't have the money to pay a shop to do the work.
Just thought I'd ask and see if anyone else has expierenced slipping in 1st gear only and the. Having it just disappear and appear to be ok the next time you got in it to drive it.

Could just be software.

That's what I'm thinking, cause yesterday it drove just fine and didn't slip at all. Ether that or I'm thinking when I serviced it with dexron vi the Dex vi is cleaning it pretty good and a valve or silanoid got clogged up for awhile and un-clogged.