i have an idle problem

[sideways-24-7]

ive been driving my 93' 240sx for a while with the check engine light on for a while now and the car was always running lean. ive put about 30,000 miles on it since bought it (2 years ago). now it has about 194,000 miles. ive been doing a little spirited driving lately and i went to get my car smogged and had to adjust the timing because it wouldnt pass. i adjusted the timing and the check engine light disappeared. i did a little redline-check(raced down the freeway on-ramp). my check engine light reappeared and when i got off at a light i realized the car was idling really rough. it would idle above 1000 rpm and then drop to 700 and then drop really low to like 200 and jump right back up. the car never stalled because i revved the car when it felt like it was going to. the problems have been intermittent but i really want to get this thing figured out. the "problem" is gone now and the check engine light is gone.

my guess would be that i need to replace the timing chain. im not sure how often im suppose to replace the timing chain so if anyone can shed some light on this issue thatd be great.

still, the problem is the idle. ive checked the tps and i thought that was why the cars running lean, put my hand over the intake and the engine shaking disappeared. ive swapped mafs with a friends 240 to see if it was the maf, wasnt. im running out of ideas. can anyone help me with this issue?