strange hesitation

[Jason Puckett]

My '93 SE-R keeps having this hesitation problem.  I'm slowly trying to track it down but its very odd.
  I'm cruising on the freeway, 3000-3500 rpm.  All of a sudden I feel a little bucking and hesitation.  Almost feels like I'm driving into a 100 mph headwind.  The car stumbles like this for 5-10 seconds then it goes away.  It'll do this every few minutes while cruising at freeway speeds.  Sometimes I get it under 'spirited' acceleration but not always.  In those cases it seems to appear around 4500-5000 rpm and only lasts about 3-4 seconds.  I'm stumped.  It seems like its something like a valve that is trying to open or close while cruising but its sticking.  Is there something that does this, opens or closes while maintaining a constant speed?

  I've tried running a fuel system cleaner through.  Replaced the PCV valve.  Going to check the vacuum hoses, replace the plugs/wires/distributor, replace the IACV.
  Maybe I should check the MAF ground too?  Could it be the fuel pump dying?

  any help is appreciated,
  Thanks

"I can sit in a ballpark after a game and love looking at the field.
Everybody's gone, and the ballpark is empty, and I'll sit there. I sit
there and think, 'Is this as close to heaven as I'm going to get?' Or,
'If I get to heaven, will there be baseball?"
--Kim Braatz-Voisard, Silver Bullets' center fielder, 1997