Transmission sensors?

I have a 1998 daewoo leganza 2.2l (I think) auto... for 3 months now it has been slipping into 4th gear at random times. even if Im stoped at the lights. I took it to my mechanic today because also the check engine light is always on. he said it came up with two faults on the computer. one was the computer itself was broken and the other was the rocker cover manifold...

Because Of everything that it has been doing I asked him if the rocker cover manifold effects how the car changes gears. he said not at all. though he did sy if I got the transmission serviced then it would most likely fix it... so got that done today. and it hasnt changed a thing. still goes into 4th gear when it shouldnt. but just not as often. (did the same thing when I replaced the computer... and everytime I have got anything done on it I have glt them to reset the computerso it doest mess with it)

I have taken it to 3 diffeent mechanics saying its 2 different problems. but no specific answer of what is wrong...

My question is, is there anything called a transmission sensor??? are there any other sensors that affect the transmission or when the car changes gears??? Is there anything else it could be after I already got the transmissiom serviced???