1992 es300 misfire when idling

[alecxjk]

So basically here is whats happening. I own a 1992 es300 with 112,000 miles on it. I bought the car about 3 months ago with 105,000 miles on it. All in all its a pretty decent set of wheels for a poor 22 year old college student, however it has a couple of problems. The primary issue is this. When I stop at a light and the car is idling it will idle at about 750rpm, however after idling for a couple of minutes it will drop to about 500rpms, and this is when a single misfire occurs. The misfire is completely random, and sometimes doesn't happen at all, however i noticed when the A/C or heater or just the fan for the vents is on, the misfire seems to be more consistent. The car does not misfire while driving, only when idling or after freshly shifting into gear. This problem is driving me crazy, Ive replaced the O2 sensors, the spark plugs, and got a complete tune up done. There is no check engine light. Ive done a ton of research and from what I can tell it is either the ignition coils failing, or something to do with the fuel injectors. But before i shell out more money that I dont have I was wondering if anyone has any opinions on the issue or has experienced a similiar issue themselves? Also i feel it neccecary to mention the transmission is slipping, not all the time, but sometimes it does slip, im taking it to get looked at next week. Its a great car and honestly Id hate to get rid of it so soon.Thanks in advance for anyone who responds, i really appreciate it. Also I read on here that i should be using 91 octane fuel instead of 87 is this true?

[jamesm182]

Is your car new enough to have a diagnostic port you can plug into and check for error codes? I would think the transmission slipping could be a bigger/more expensive job than the misfire in my opinion.

Premium fuel is generally a bit better in that it also uses detergents too, so it can help to keep everything clean. Personally, if you're having problems then using better gas might not be a bad idea anyway just to see if it helps.