Nissan Repair: my maxima heater core clogged, nissan maxima se, 2004 nissan maxima

Question
i have a 2004 nissan maxima SE automatic drives prefect. tonight at a stop light i heard a clunk sound then it shuts off. i turn the engine back on and put it in drive and step on the gas. the car starts struggling to move i barely made it home going so slow. ok i try to use the stick siff. but when i put it in stick the gear was stuck in fifth. supposedly to be in first gear. i dont know why that happen. my brother came by and look he sayed i need a engine flush because the heater core is clogged. and when the heater guage goes over the middle the car is going to shut off.  one other question why when i try to go in drive the car slams also in reverse its like the engine is going to jump out the car each time i go in those two gear.

i know my question kinda long but i dont really know bout cars so  sorry for poor english

Answer
take the car to a professional mechanic.  the people that have been giving you advice, you should avoid their advice in the future.  it sounds to me like you are low on transmission fluid to be honest, but it could be mechanical.  you need someone qualified to go over your car and check the basics.  sorry i couldnt be more of help, but this is the best advice i can give you.  good luck.