Happened by this thread while just reading through the forum, registered so I could give my 2 cents.

This was an issue I had with a customer car. Fixed a misfire from cracked spark plug, took it for a test drive and had no guts until after ~3k. Checked timing, and timing mark was jumping around like crazy. Thought cam timing was off from a previous job or skipped tooth, so I pulled the valve cover. The belt was hanging off the back of the cam gears by about 1/4" and the timing cover was soaked in oil (uh oh). I ended up finding that the woodruff key on the crank/pulley had gouged out a chunk of the crank, basically the crank would spin but the pulley wasn't catching up until after, leading to way off cam timing. Basically the crank has the ability to speed up and slow down every revolution without the cams being in sync. Exh/int are getting advanced/retarded at different times, and not advancing or retarding together, so you never have a reliable base timing. At idle, it doesn't really do a whole lot out of the norm, but when you request power, everything's all wonky. This was after checking everything else. If all else fails, check the crank. 1.6's have this issue more than you would think.

edit: also, check to make sure that the pulley isn't wobbling back and forth/side to side while idling. There is a huge known issue with crank pulley bolts in 1.6's (also 1.8's, but not as many, yet...)