intermittent weak flush on toilet

We have two toilets on the second floor of our house. One toilet has frrequent but intermittent weak flush. A few flushes in a row might flush strongly, then one, or maybe two,will be weak. The water in the toilet never quite gets beyond a rapid swirl. And then there's another strong, complete flush. Mind you, this is with nothing but water in the toilet. When I say "in a row" I mean simply letting the toilet tank fill completely and then flushing the toilet after each fill.
We've had a plumber use a snake to make sure nothing is lodged in the "S" of the toilet. He said it could be a bird trapped in the vent that exits the roof, and that if we bought a new super-dooper water-pressure toilet from them ($900 installed) they'd run a snake up towards the rood and down towards the sewer from opening in the bathroom, at no extra charge. He lifted the tank lid to look inside, and noted that some bubbles were coming up into the tank but did not explain the significance of this symptom. What do bubbles coming up into the tank point to? Is the toilet not seated properly on the lead pipe in the floor?
Thanks Timo
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Timo wrote:

Are you SURE the tank is full prior to each flush?
Try leaving the tank lid off for a while and eyeball the water level before you flush.
Have you reamed out the water discharge holes under the rim of the bowl with a small screwdriver or a piece of coathanger wire?
When you get that "weak flush" does the water level in the bowl rise unusually high?
Give us a bit more info and soeone will find the answer for you.
Jeff
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Jeff, Confirmed, the water level in the tank does rise to the fill line. Yes, the water level on the weak flushes does rise unusually high.
I have not reamed out the water discharge holes under the rim of the bowl yet, but will do that and report back.
Thanks for the help. Timo

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Timo wrote:

Even though the plumber ran a snake through the S-bend of the toilet, there may still be something solid in there like a jar lid which can get pushed aside by the snake, but remains there to flip to it's "closed" position just whne you think it won't happen again.
Even though it's a bit of a pain, I think you might have to pull the toilet up and lay it on its side so you can see if there's anything in the S-bend from the bottom end.
You'd also be able to see if there's anything funny lodged in the elbow bend under the toilet.
The bubbles in the tank you mentioned are curious. If they are still there can you see where they are coming from?
Jeff
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Fixed. I replaced the wax gasket where the toilet meets the pipe in the floor. No longer a weak flush. Air leak must have been interfering with the creation of the siphon? Timo
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no, you probably dislodged a toothbrush, plastic toy, comb, or favorite is -those small soaps the ladies keep in a basket on the lid
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