Backfiring and stuttering when cruising

When I'm driving at any usual speed, it started happening between 30-40mp/h in 3rd gear but when I switched to only 4th for 30-40 it stopped, then recently it started backfiring stuttering and losing power when I was cruising at 70mp/h in 5th gear on the highway, I took my foot off the gas and re-applied my foot to the pedal and it stopped having the issue, only for about a mile or two, It is happening more and more and I've tried cleaning the IAC valve, using Sea-Foam in my gas, (it stopped backfiring and having issues like that for about two weeks and now it's happening again) and it idles roughly and the car starts shaking and the engine's RPM sits at about 650 to 900 but sometimes it dips down to about 300-400 and then shoots back up to 650-900, I'm trying everything I can right now and I need help ASAP