When the vehicle is idling in DRIVE or REVERSE my RPM's fluctuate in intervals every three to four seconds

Good day,

My vehicle has about 175,000 miles on it. I have driven the car for the last 20,000 miles and years and I have had no issues with it. It sat parked for about seven months while I was deployed overseas and drove perfectly fine for the three weeks I was back. I then left for another three weeks overseas again and had it parked on a hill. When I got back from these three weeks about six days ago the car has had several issues.

To start, most of the oil had drained out of the car and was on the pavement. I immediately refilled it and have not had any leaks since, so I am not sure if this is really part of any issue that persists.

Secondly, when the vehicle is idling in DR or R, my RPM's are starting to fluctuate in intervals every three to four seconds. It will dip down to maybe one hundred or so and then pop back up to normal. When it does this, the entire front end of the car starts to shake and shudder, and everyone can feel in vibrating inside the car. The car has stalled twice while sitting at red lights. This stops completely if I put the car in park or if I apply any pressure to the gas pedal. It happens when the car is cold or warmed up as well.

I originally suspected it was an issue with my IAC so I pulled it off and cleaned it real well, and then let it dry overnight. After putting it back in and firing her up, I got the same issue. I then concluded it must have just been bad, so I bought a new one, but same issue persists. I have been recommended to get new plugs and wires in there but some of the shops around here think it will fix it and some do not, and I would rather not shell out $200.00 or so for a tune up that may not be needed. I do have fuel injector cleaner in the tank right now and about half a tank of gas left but it has not made much of a difference yet.

Thanks for reading!