Since you mentioned that your fluid is yellow, that would mean that there is water in there. Not a good thing at all! The only place that water could easily enter the tranny is from the cooler which is part of the radiator.
Does the car run at normal temp?
Have you check the coolant res and see if there are signs of tranny fluid floating on the surface?
Sounds like it is time for a new radiator to me.
I would not drive the car at all, since the reason the car is acting up is because of the coolant in the tranny fluid. This is either causeing the valves/pistons to stick in the valve body and not allowing for proper shifting or the thinner fluid is allowing the band and clutches to slip or runaway line pressure or any of the above.
As long as you can get the cooler section of the radiator checked and verify the leakage, then you should be able to replace the radiator and flush the tranny. And hope that the coolant did not do any harm to the transmission.
Good luck!!
