Hesitation/mild surging when applying throttle at cruise speed

[rowan194]

A few days ago on the freeway I was cruising at about 80km/hr, put my foot down a fair bit to get into a better position to change lanes, but nothing happened... no change in engine RPM, no trans kickdown. As I released the accel back to the position I had it in, the trans made a clunk as if it had started changing, but aborted when it realised I'd gone back to cruise speed.

Possibly related, the next day I was again already at road speed, behind a slower car, which then cleared, so I put the foot down... this time it did accelerate, but it was a bit sluggish, and I could feel the car wavering, like there was a fat guy rocking back and forth in the seat.

Have not experienced any issues like a misfire or anything noticeable when accelerating normally from standstill, it's just when cruising and I give it extra juice.

Didn't see any leadup to this behaviour, first time was sudden as mentioned above. No probs yesterday. Any ideas?