transmission

91 sunbird le 4cyl 2.0L TBI OHC is their anywayt o get to the transsmission bolts to see if they are loose. ive posted reciently that when going around 25-30MPH my car starts to shake and now that it is summer i have window open then when it has the problem i hear somthing rattleing and 89 sunbird had told me he had found his trany bolts loose . one day going up a hil ive had the problem , the car does not have much power then when it going to 3rd it has power all of the sudden. could i be a bad flywheel or a bad 2nd gear? my dad drove my car after i bought it and cleaned it out he took it for a drive then we wnt from our house to some place like 2 miles away and we took high way and on high way he looked down and noticed that he was in second gear the whole way. could this have casued the problem. any1 knw about how much it would cost me to change transmission fluid and filter?