Auto trans problem

[16valvedave]

Could someone please explain to me how the gearbox knows when to shift and into what gear ? My car has always changed down into second gear if i kick down even up to 60 mph, which at that speed revs the nuts off the engine. I had always wondered if there was an adjustment that could be made to stop it changing into second at above 55 mph. However i have since had a full gearbox overhaul and now i have the complete opposite problem. My box will no longer select second at kickdown above 40 mph, it will only go down one gear into third. As my car produces great power in second between 40 to 60 mph, i want my box working properly. I took my car back to the garage that overhauled the box and he is not sure what going on. I have now fallen out with this guy and the problem remains.
Has anyone got any idea where i should try to look first. I am a fairly competent DIY'er, but have no experience with autos.

I must add that i can manually select any gear with no problems but that is not the point. I want everything to work as it should.
Sorry for the long post.
Thanks
Dave