Surging power on the highway
While on the highway at a steady speed and constant throttle pressure the car will all of a sudden act as though additional power is available and will start gaining speed. Then a minute later just as suddenly it will 'turn off' and lose that slight boost. The RPMs do not change.
Are you perhaps switching between in/out of overdrive by resting you hand on the shift lever? There is an instrument panel light to show if you out of overdrive. There is a button on the shifter to toggle overdrive on and off. So, for example if you need to climb a long hill, it is better to turn overdrive off so you have the torque you need to do that smoothly. In other situations have overdrive ON for best gas mileage. Good Luck HTH
Could be the computer changing timing due to knocking, but it's not bad enough to throw a check engine light. May want to try some better gas or a can of Sea Foam in the tank to clean the engine and fuel system out a bit. Might also be a dirty fuel filter, 1999 it's about that time...
I currently put Cenex 89 octane in her. And no I'm not pushing overdrive, the 1MZ has plenty of torque to get up big hills without downshifting anyway its quite nice. I use a fuel additive once a month or so and oil additive with MaxLife blend on a 5000 mile interval. The throttle body is clean and it has fairly new spark plugs. The transmission fluid was changed not too long ago as well (it shift normal) not velvety like a new camry but much better than my old Jeep, and the torque converter always locks at about 42 mph, it does shudder very slightly (very few people would notice) when it unlocks from 4th under acceleration but i consider that quite normal? Im extremely adamant about using my emergency brake before putting it in park. The engine doesn't knock. It also has a new PCV valve. Im wondering if it could be related to the EGR valve?
Is it the A/C compressor kicking on?
No, its not that often.. and id say whenever it happens its a good 15% power increase
I would try 4 quarts of Royal Purple Max ATF for the torque converter issue, and you will get that velvety back. If you've had a filter put in not too long ago, I'd drain the transmission oil pan and measure what you took out. An old 5qt oil jug is handy for this. Should be about 4 qts though. No codes though? Also the only fuel additive I would trust is Sea Foam. It's made from Pale Oil, Naphtha, and Isopropyl alcohol.