91 Toyota MR2 automatic transmission when i full stop sometimes i step on gas RPM goes up but it feels like it`s still in neutral, i have to down shift before moving again. I can`t find what`s wrong.

Does it stop if you step on the gas pedal lightly?

putting it into a gear and hearing a knock indicates that the engine mounts or transmission mounts are broken
naturally if you put it in neutral it will not move as that what neutral is
perhaps the neutral position needs adjusting as it may still be in drive
have the fault codes read to determine any sensor/servo/solenoid faults before spending big money on a transmission

Transmission low or transmission bad

It sounds like it is kicking out of overdrive. Many code scanners sold by auto parts retailers will not read manufacture transmission codes. I had this issue with my Ford Contour. The o/d light would flash but I couldn't get any codes from it with an Actron scanner, which is similar to what you find in parts stores. I used a tuning device (SCT Xcal) loaded with Ford software and that pulled the codes for me. If you are getting a VSS code, that is a good place to start. The computer uses this sensor to know how fast you are going so it can select the proper gear. If it doesn't know how fast you are going, it tries to guess based on your RPM. It wouldn't surprise me if it got stuck in first gear. The exact VSS code you get would help but I think you are going to need to replace that sensor.

Have a pressure check done on your fuel system.

Don't listen to that guy, he doesn't know what he's talking about!! I have an 85 Celica with the very same issue. I was told it could be some kind of temperature sensor that may not be working properly. There is a sensor, I was told in the exhaust line that indicates that the car has reached operating temperature and until that sensor is tripped, it will not activate the solenoid that allows overdrive to engage, it could also be that solenoid malfunctioning, or maybe a fuse or the switch that turns on the overdrive malfunctioning.. I would check those things first before you go thinking the trans is having issues.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/keith_1e46710d649c2ed7

is your ECT Power switched on this causes higher revs

20 year old vacuum hoses are done. Replace all you can. The little plastic ones too. They control everything, transmission, computer, fuel, brakes, everything.

Sounds like you need gearbox oil (transmission oil in USA) .... check your levels and top it up.

Go ahead, get the TPS close, we'll final adjust it later.