1990 Toyota 4Runner Lack of top end acceleration

When I purchased this used vehicle it drove fine in town. I then found that out on the highway I had to run full throttle to eventually reach 50-60 mph and the fuel consumption is outrageous (1/2 tank in 70 mi). The injectors were leaking so I replaced them, and thanks to your advise I have gotten the vacuum lines put back correctly. I also replaced the timing belt (and have rechecked the position of the gears 3 times), replaced the fuel pump, fuel and air filters, spark plugs, checked the resistance in the spark plug and coil wires, air gap in the distributor, and have set the timing. I have found that while watching the timing mark with a timing light as I accelerate the engine, the mark will begin to advance but around 2500-3000 rpm it starts skipping back and fourth and the engine begins to cuts out and pop back. What controls the spark advance? I have thoughts on the TPS being a possible problem but am reluctant to keep throwing parts at the problem. Any advise would be appreciated.

updated 5-27-10
I donated and I did get a reply but it doesn't seem to relate completely to my question. The main body of my question is "what controls the spark advance on my 1990 Toyota 4runner w/3.0 L V-6. Please re-read my question and reply if you are able. Thank you.