Idle issue

I have a 2004. Corolla S with a 1.8. I bought it from a Toyota dealer with a 120,000 miles on it 8 months ago. It's now at 135,000. When car was in park, it would idle at 900rpm and drive 750rpm. It started running rough so, the plugs were changed, and it went smooth again for about a month and a half.. Now, it's running rough. It fluctuates when I put it in park to 1100rpm back down to 750/800rpm. In drive at idle its at 750rpm but, idles rough. I've checked the I.a.c valve, throttle body, air filter, and engine mounts. My fuel mileage went from approximately from 300 miles a half a tank to about 250 miles a half a tank when topped off . A/C wasn't used. I also fill up when I get to half a tank. Any insight would be greatly appreciated.
Hi, my very first post here in this forum. It's nice that you've check your iacv, TB and others and what have you found out then? Are they still in good shape? Anyways, checking your MAF sensor and cleaning it might be necessary already, pcv may be consider also.
You may check it in YouTube with Peter Finn or Mr. Scotty, just don't forget that you would only want to use a CRC MAF Sensor cleaner or equivalent.

Mine had issue like yours an 02 corolla with 230kKm, I tried those cleaning this repalcing that then none works well tho improve ans now I do think rough idles at my corolla mainly fault by me. I used the wrong grade which was genuine 20W-50 regular oil a year ago couple of times and that really hurts my engine I think so and as I observed. Now I've been using toyota 5W-30 where as the engine was absolutely desiged for. I also don't use engine flushes.

I'm still observing our corolla, I just almost ruined the CHain or engine by replacing an O-Ring a month ago, hope still to run more thousand miles.