No Acceleration Power - Pain in my Side

Hello all,
I have a 2006 Nissan Xterra 2wd that’s having…a lot of problems right now. My biggest? There’s not much acceleration happening - especially uphill. I’ll top out at about 30 mph on a straight road, but a bit of a steep hill, I’ll put pedal to the medal and won’t get above a good 15 mph. The car idles high - so I thought it was a vacuum leak, apparently not. Multiple random misfires. I changed both spark plugs and ignition coils - but when I was heading back to college from home, more of the same. Took it to a mechanic last week, and got a call back today saying that even he can’t figure it out. Please tell me what I’m missing - I need this car.