engine problem with 99 Nissan Maxima

I have a 99 Nissan Maxima with about 130k miles. I am having an interesting engine problem. A few days ago, a few blocks from my home
at a stop sign, the engine sort of shook up and down a bit. The car's front gently moved up/down, I let go of my break a bit, and it went away. Didn't happen again for the rest of the drive. I didn't know what it was and almost thought maybe it didn't happen. ;)
This morning, a few blocks from my home, it happened again. I saw the rpm needle shoot up a bit (maybe it went down a bit before it shoot up) and the front of the car shook gently up/down again...
What could be the cause of this kind of symptom? The Maxima is known for its defective coils, and I had all 6 replaced (5 from nissan, 1 different-brand from mechanic) 2 years ago. I also replaced the spark plugs at the time (6 Nissan OEM platinum plugs), at about 115k. Could the non-Nissan coil acting up causing this problem? Could the spark plugs causing this issue? The service engine light is not on.
When the coil did went bad last time, the engine was vibrating gently up/down, but once the sympton happened, it never went away until the coil was replaced.
Another symptom that may be related to this, and has been happening for a while, In the morning, when I first start to drive the car. During the 1st long acceleration on a local street, I noticed the engine sort of vibrate a tiny bit (at a higher frequency compare to the idle at stop vib). It's very very subtle, but I feel like there's a slight lack of power compare to what the car was like before. After the engine warms up a bit, it doesn't happen anymore. I've learned to accelerate more gently initially.
I just changed the OIL, Mobile1 10w30. Been using mobile1 since 60k and the engine's been performing great, until recently...
Any help/pointers will be appreciated. I am not a car person, so unless it's something trival (like replacing the plugs/coils), I'll probably have to take this to a mechanic.. but I want to know what might the cause before I do.
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Raymond wrote:

Maybe there is water in the gas.
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That is a "brake" , by the way. This is a little off the wall, but I have seen cars shudder when the brakes were slipping a little bit. If they slip when being applied rather strongly, the whole chassis can oscillate. If it is something like this, it will not set an error code on most cars.
This may not be what is happening in your case, but it would be difficult to diagnose this without being there to see and hear it.
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wrote:

I've heard of the engine mounts on them going bad. If the engine is going 'up and down' I'd wonder if the mounts are bad. That could also cause the vibration. I think the mounts are very expensive silicone filled gizmos. If my memory is right, when this happened to a friend of mine it cost him over $1000 to have them replaced.
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It's interesting that you mentioned the engine mount. When I was at the dealer for some minor fix the other day, they said one of the engine mount was bad. I took it to my mechanics, and they said the engine mount is fine...
Why would a bad engine mount cause the engine to vibrate up/down slightly, but just once? Thanks.
Raymond
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Raymond wrote:

A bad engine mount will make all sorts of normal (or slightly abnormal) engine vibrations seem much worse. Things like the A/C compressor kicking on will now be more strongly transmitted to the rest of the chassis. Same for minor misfires.
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wrote:

I'd only be guessing. But it might be that it just needs a hiccup to get it to jump and it only hiccups once in a while. It's very possible that they can look ok and still be bad because they are not just a chunk of rubber like on an old V8. They are often fancy smancy silicone fluid filled gizmos specifically designed to filter out the frequencies of the engine at idle that would otherwise make the car vibrate like a Toyota going uphill with a tank of bad gas.
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If a visual inspection shows nothing, and the coils all ohm out the same, it's time to get a scanner on it. My guess is a code P1320 (ignition primary) or a P30X (misfire) are lurking in pending codes. Just had one like that last month, installed 6 aftermarket coils, ran worse (2 were defective out of the box)...finally fixed it with a set of used oem coils from the junkyard. Intermittents like this can nightmarish... HTH, Ben
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