I have a 1998 nissan altima gxe 2.4L engine. I bought the car a month ago and it ran fine at first same some transmission slips. After it hit 200000 miles I start having trouble. It won't shift into 3rd gear and every time I try to get it to shift it just peaks at 5000 rpm and starts sputtering. I changed the oil and gave it a tune up(new distributor, new rotor cap, spark plugs, boots.) after I did that it starts running worse and I had to adjust the throttle cables because when the car idled it stayed at 2000 rpm but after I adjusted the cables it worked the same as it did before. So I decided to change the oil again(and the oil filter).and I checked the transmission fluid it had bubbles on the dipstick. This is not normal I was wondering what is wrong with the transmission, I know I probably need to get it flushed asap but I was wondering if maybe it could be a vacuum leak or something? And also driving at a constant 5000 and up rpm will mess up the engine won't it? What do I do...

Try changing the transmission fluid with Mobil 1 Synthetic Multi-Vehicle ATF. If this doesn't help, third gear's clutch pack is likely shot and you're looking at rebuilding or replacing it.

Its been a while since the fluid has been changed as well