Very possibly a vacuum leak. I had a similar issue. I could never figure out where the problem was. I checked all vacuum lines, and no leaks. It turned out to be the throttle body. I stripped it apart, cleaned it up. I was too cheap to by a gasket set for it so I silconed the whole thing, and that fixed the problem. I figure the vacuum leak was caused by one of the old (91 tracker 385,000 kms) gaskets in the TB.