Engine pinging but not always...

I have a 2010 Prius which I bought used. The car mostly runs and drives great, but every now and then it would have a slight pinging. I had the spark plugs changed at one point and a couple of other things done and didn't notice it again for several months. Then a couple weeks ago I noticed it again when I was accelerating from a stop. A few days later into British Columbia and my car didn't love the mountains at all. The steeper climbs would start it pinging and it seemed sluggish. When I finally returned home I took it into the shop and had them check it out. They said that everything looked good and they couldn't recreate it. One guy at the Toyota service center said he was pretty sure he knew the sound I was describing. He said it wasn't harmful and they could get a filter or something to fix the problem, but it wouldn't be a problem.

Today my car started doing it again when accelerating from a stop. I had read in forums that sometimes it happens if your car isn't getting enough gas and to give it more gas and see if it goes away. That was definitely not it, because giving the gas just made it worse. It continued to ping when i accelerated until i made another stop and let the car sit a couple minutes. It hasn't happened since, but I know this isn't supposed to be happening.

I have assumed it was a misfire from the beginning, but I don't know for sure. I really don't know why it would only do it sometimes when accelerating and not others. Would this be a valve problem? Bad plugs? Bad wires? I would assume that the Toyota service center would have caught it if it was bad plugs, but who knows if they even checked them.

I drive for a living and this is my work car so I could really use any help that I can get.

Just a little additional info... the car has 205,000 miles on it which is barely above half of my cars usual life cycles. I don't know if I can push this car as far as I usually do, but I easily got my 2004 Prius over 400,000 miles and never had any trouble with it at all. I get all maintenance done on time as well.

 

Thanks for the follow up question Raytheeagle. This car had 115k on it when I got it. I put around 100k on a car in a year. It didn't come with maintenance history, but i have had it serviced many times including an initial inspection and every time i am told it's in great condition. The plugs were done about a year ago... i would guess about 80k miles ago. No work on the air intake or the egr as far as i know. I had the manifold cleaned once because it was running rough at cruise speeds and it took care of the problem. The car does consume oil which seems to be pretty common for 2010 prius. I would say 1 quart every 2,000 miles. I purchase whatever gas I am able to get, but that is usually from respectable places. By top tier do you mean premium or just from top tier stations? I was told to stay away from premium with my Prius but I don't know whether that was good advice. I just use regular and pretty much always have. I have not used fuel injector cleaner, but i could give it a try. Would a fuel injector cleaning fix a problem that only happens about 5% of the time?

Also I have had it just over a year. It was a low mileage year and I only put about 90k on it so far.

 

I have pretty much read everywhere that I shouldn't be going for Premium with a Prius due to it having high ethanol and not being necessary. Should I be aiming for a certain octane number?