ck400

I own a 93 sc400. Im having a couple of problems after repalcing my mass air flow sensor. The car now hesitates and stalls, the other problem is when i ill mash the pedal to the floor the car revs to aprox 4k rpm and refuses to shift . Every time ive taken this to a mechanic the codes decide to disappear and evrything runs fine . i have no idea what to do