RX8Club.com - 04 Auto rx8 is giving me problems need help

Ok, my car has recently started to give me issues at start up. Some times when i leave my car in my schools parking lot, then come back 8 hours later to turn it back on it gives me a long start up and tons of vibrations from the motor until it does finaly fire up.

But yesterday was the worst Ive ever seen it do, I went to turn it on for the first time in the day and It did the same thing but this time it almost didnt want to start up and when it did fire up, It went to 1,250 rpms and started to lower to 500 then back up to 750 with tons of engine vibration and it was making some wierd noise and the engine cell was blinking. Then i decieded to take it out the drive way and it seemed fine when i put it into reverse and then backed up. But when i put it into drive and moved it alittle it went from normal 1,000 rpms to like 300 and then seemed to do this 2 times with tons of engine vibration and also engine cell was blinking. But then i started to put some gas in it and it setteled out and the engine seemed to act normal.

Now today it started up normal and went to 2,250 rpms then setteled down to 750 rpms like it normaly does at warm up, it also had a solid engine cell and i took it to get and oil change and it didnt have and engine cell showing up. Then i cleaned the MAF sensor to see if it would make a difference, but it didnt and then after i started it up and it seems to take a while to start up, not like mormal start up. But yea thats my problem i hope some one can help.

Btw it has 38,973 miles on it and its an 04 auto and I bought it a year ago with 29,000 miles and i checked and saw that the previous owner had an engine changed but i dont recal at what milage. I also went to auto zone yesterday when the problem was really serious to see the engine cell codes and it was reading a couple miss fires.