No leaks in the exhaust before o2 sensor is there? Probably not but it the only thing I could come up with. Exact same thing was happening to my buddys 94 22re. His problem was that his o2 sensor was not tight agains't the rednecked flange that a muffler shop installed. My next move would probably be to replace the injectors. How many miles on the motor?
Rob

"4X4 in upper case is $X$" Quoted from http://4crawler.com/4Runner.shtml
'00 tacoma 4WD---Sold to Toyota
'93 pickup 2WD
'85 pickup 4WD