91 940GL B230F and AW71L

I've been experiencing a hard shift from 1st to 2nd gear in my 940's transmission. I have replaced the kickdown cable and serviced the Valve Body (I'm the one who did the website for that).
Problem is: it's STILL shifting hard! But only when the throttle is less than about half open. Under harder acceleration the shift is very smooth. In fact I can barely feel it. Why would it still be slamming into 2nd gear during small throttle openings? All the other gear changes are fine. The car had a fluid flush back in February. I did it again about 2 months later. Then the valve body was done which required about 6 litres to fill the trans up again. Then the kickdown cable was done - needing about 4 litres to fill it. So the fluid is very clean. Would a synthetic solve this? The only synthetic I can find around here is Castrol Transmax Z. Mobil 1 synthetic fluid is available by special order but at $352 (Australian) for a 20 litre drum I can't really justify that.
One shop I took the car to only wanted to rebuild the entire transmission for a cost of about $2000. I can get a Volvo exchange transmission for $2500.
Does anyone know what the problem is and how to fix it? Should I just replace the trans?
Thanks for any insight

Brad