Engine misfire while accelearating at highway speeds

My 2002 Alero 3.4 with 200k has an odd misfire while accelerating. It only happens at 70kph to 85kph and only when the is a slight increase in speed or going up a hill and only in the above speed range. I changed the plugs and wires. Thinking it was a bad wire or plug.

Any ideas what it might be? electronic or mechanical? A coil pack? gas mileage is not great either. Also, as it is happening it feels like it is bucking a heavy wind and the tach will bounce up and down by 200 - 300 rpm. Also, it is not consistent. Some days it will not happen.

do you have access to a scan tool that can view freeze frame records and live data? This would help narrow things down.

At those speeds (if my math is right) that's right where the transmission is deciding to go into overdrive. Once it's in OD, the transmission will decide to downshift if you push on the gas to accelerate. When changing gears (or coming out of OD), the torque converter will unlock, and then lock up again when the new gear has been selected. Your transmission may be having problems with the torque converter, or with the OD band. Get it scanned with a scanner that can read the GM transmission codes (typically not the freebie scanners at A-zone et al.) You could try re-seating the electrical connector to the transmission (front driver's side, little hard to get to).
Also, check trans fluid level and whether it needs to be changed. See:
http://www.automotiveforums.com/vbulletin/showthread.php?t=630214

Hope this helps.

Also, I just remembered the time I had somewhat similar symptoms. I could be driving down the highway, in OD, rpms ~2000; then when I went over a slight bump, it would come out of OD, the rpms would kick up a bit, then it would go back into OD and be OK till the next time. Turned out to be the brake switch, which on my 2000 Alero is a mechanical switch up under the driver's foot area, which is mounted against the brake pedal. When I'd go over a bump, the brake pedal would trigger this switch for just an instant, but the computerized transmission is smart enough to unlock the torque converter when it thinks the brake is applied. I just took out the brake switch and cleaned the contacts, and re-installed it to where it would function properly.
If you decide to check out the brake switch, be aware that there is another similar switch, also near the brake pedal, which turns off the cruise control when the brake is applied. (at least there's one on my 2000).