hey i need help with my new car

[coolcuban101]

ok so i bought a 91 240sx a few weeks back and the problem is now that when i shift into third gear it makes a grinding noise, and its only third gear. also when i am still or in nuetral my car will start idling up and down and a lot of times just dies. idk what to look for or what to do. im kinda new with cars so please bear with me.

[bone_stock_240]

I think the transmission problem is just your synchro wearing out. Probably be easier to replace the whole transmission. The rough idle may be your idle air control valve. that may need cleaning. Also might be a maf problem