I think that the first thing you should do is to scan the TCM and ECM, if it's an electrical issue it will show up there.
Check ATF levels.
Those are the basic things you should do before attempting anything else.
Gear slipping or hard shifting are common failures on these units, search this forum and you will find several issues like yours.
If you do not have equipment to scan your car's computers, then once you take it to the 2nd shop ask them for written results of the scan and post them here.