350c lockup Problems

[Jackie Mac]

My trans. is shifting in and out of 2nd to 3rd gear, sometimes wants to stay in 1st to long. I checked the vaccum lines aand replaced the vaccum module on the back of the trans. which did not help. I hear that the vaccum device( sorry I don't know the name). that sets in the engine compartment in the center of the firewall with a vaccum line controls the lockup feature on the trans. Could this be the problem? Does anyone know the name of that part? Any ideas on the cause would be appreciated! Thanks much Joe