My 87 was doing about the same thing. It occured at about 3000 rpm, only in 5th gear and only when going up hill. I did the same as you with the plugs and all and nothing helped. One suggestion from a club member that did actually help was to remove the air intake silencer and remove (cut out with a very sharp knife) the restrictor tube from it. This will increase the size of the air intake passage by about three times the size it is normally. No promisses, but it sure seemed to help mine.
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