Strange problem with 240 and dont know what was wrong

[831_salinas240sx]

Model: NissanYEar: 1991 FastbackEngine: KA 24 DE (bone stock, stock everything. you name it, its all stock)Color: Metalic Grey transmission: ManualIssue: Idle Problem and transmission Problem

heres what happened, I turned on my car like usual, and i drive it to pick up my girl. When I am in the freeway, going down hill i usually put it on neutral and let it coast its way. so as i put it on neutral my idle starts going crazy, it goes up tp 1000 RPM then quickly drops down to 100RPM then jumps up to 900 RPM and it basically does this for 30 seconds after that it finally falls below100 RPS and then stays off. when it turned off, i put it in gear and jumped it back on like i was taught to do. when i got back home, i checked the muffler because i would notice a little smoke coming out, so i assumed it was probably because it was cold outside. so when i told my girl to press the gas and i checked the smoke, the smoke was a little blackish and the RPM's would still bounce up and down. i had to stop by pressing all three pedals the clutch with one foot and the ges and break with the other. eitherway, after i got tired of it, i blew out the dirt the filter had, i checked the fuel cap, and i checked the oil. everything was ok. but i noticed that when i was closing the fuel cap, it was a little loose. everytime i would check it after that it would be loose. then one time i got fed up with it and super tightened it. after that my car worked perfect. And i also experienced quite a number of back fires mainly at 3000-3500 RPM ( a lot of back fire description: "<"= pull back ">"= driving forward one minute my car is > > > > > and then it starts doing this: > < > < > < > < > > > > > < < < < > > > > < < < > < > < > ) so my question is this:

Can the fuel cap affect the engine if not on tight or what could be my issue?

and another issue is my clutch. when i am shifting at high RPM's from 1st to 2nd (6000-6500 RPM'S) at times, my clutch has problems gripping gear 2 it grips but then the RPM's show that it is slipping down to 3000 RPM's for some weird reason. and in other ocasions i feel a grinde. the grinde occurs when i shift ater 4000 RPM's and only when i shift from 1-2 and from 2-3. and its not a wrong shift grinde, its more if a spring not being separated enough type of grinde so if there is anyone that can help my out in any way please let me know.
Modified by 831_salinas240sx at 5:11 PM 9/26/2006

[smokin240]

Grinding while in gear is your transmission. Slipping while in gear is your clutch. It could be a number of problems causing your wacky idle, but first thing that came to my mind was TPS and / or MAFS. Good luck.

[831_salinas240sx]

In reference to my transmission, and my clutch, i was thinking of just changing the clutch out and geting an exedy stage 1 clutch so that it will grab more. but when the mechanic take out my transmission to change my clutch, i'll ask him for his opinion of the condition to decide wheather i'll have that changed out as well. I am sorry, but i am not good with abriviations. Can you please explain to me what is a TPS I understand that MAFS refers to the Master Air Flow Sensor. Right?????