1988 Toyota 4Runner Performance problem

My 1988 4runner recently started to have engine performance problems. I have had to down shift more and more when going up relatively shallow hills, and more recently, while driving at 70 mph, it began to decellerate despite me not taking my foot off the gas. I could step on it some, and it just continued to go slower and slower until it got to about 45 mph. I would then downshift into 4th and bring the rpms back up, and sometimes this would "work" and allow me to continue at 70 mph, and other times I would have to do this several times beofore I could maintain 70 mph. When it was slowing down, it would get to the point where it would begin lurching something terrible. In the last 10,000 miles I have changed the plugs, cap, and wires, and filter. Any suggestions? Thanks in advance for any help you may provide.
Cheers,
Mike