Ok... let's start by looking at what causes the 1-2 shift in the 4R70W to start with.... and then we can work from there. The fact you have an intermittent is actually a good thing because it rules out certain mechanical failures that once happened would not ever be right again without a rebuild.
In the A4LD and 4R44 and 5R44 series the 1-2 shift is caused by applying the intermediate band (in the 5R55 2nd gear is nothing more than an overdriven 1st gear). But you have the 4R70W. It gets 2nd gear by applying the intermediate clutch, located directly behind the pump and using the case as its housing. Your intermediate clutch is applying late, sometimes. Why? is the question.
If you did not get a 1-2 shift at all or if it was always screwed up we could be looking at a myriad of things, but the fact that it works fine *sometimes* means we can look past those things... So let's look at the hydraulics, as it is likely something somewhere in the hydraulic circuitry not always supplying the force to apply the intermediate piston and locking up the intermediate clutch and effecting the 1-2 shift.
First off, this transmission has 2 shift solenoids, housed together on the main control unit (valve body) and easy to replace by merely dropping the pan. When the PCM sesnse the conditions are right for the 1-2 shift (more on this later) it turns off the SS11 solenoid off, and the SS2 will (or should) remain off. By turning off the SS1 solenoid, pressure on one side of the 1-2 shift valve is removed and so the other side overpowers it and it starts to move. The movement allows fluid to start to energize the Intermediate clutch circuit. This circuit applies the intermediate clutch and causes the 1-2 accumulator to become active. Important in all this is that there is a check ball involved which seals the circuit and allows it to pressurize. A stuck or sticky check ball wouldnot allow the circuit to pressurize, and you would not have a intermediate engagement due to that fact. There is also fluid flow to the OD servo regulator, but I think for our analysis at this point we can put that in the back of our minds.
One of several things is likely happening. First, the PCM is not getting the inputs it needs to command the shift. These inputs come from the output shaft speed sensor (as well as others but that is the main one) for this shift. Possible intermittent OSS problem? On the list of possibles. Cost to replace? $20 plus labor, on the outside of the case near the rear.
If the PCM is telling SS1 to turn off, is SS1 hanging up? Another possible. Cost to replace? A bath in ATF when you remove the pan and about $35 for the solenoid pair. Add a pan drain plug.
If the PCM is doing its job, and SS1 is doing its job, is the 1-2 shift valve sticking in the VB? Is the 1-2 accumulator causing a problem? Is the check ball grunged up in the main control unit and not seating causing a problem? The 1-2 accumulator is under the main control unit, and if you remove it SHOULD be replaced for under $10. The main control unit should be gone through to check for any debris or other problems that might cause either the 1-2 shift valve to hang up, or the #8 checkball not to seat properly. Add new gaskets when you replace and torque it properly using an inch lb torque wrench.
Obviously these things MAY not fix your problem, but these are the items that I sense are involved, either ONE of them, or a combination. Good luck.
