transmission acting up

My 95 T-bird transmission is shifting from OD to third gear and back,while driving at highway speed.Can someone tell me what the problem might be?

See my reply to ( 94 t-bird trans.) by ( 55mainline ) MLPS= Manual Lever Position Sensor ( Neutral Safety Switch ) mine was searching at lower speeds AND down shifting as high as 75 m.p.h. Located on driver side of trans. and $35.00 at any parts store. Good Luck. E-Mail If need more. Roger