Got an 88 civic, d15b2 engine

Last 2 days, its started having very lumpy running. Its shakey at
idle, and feels like its missing yet the check engine doesn't come on
(as it does usually when it misfires)

it appears to have a loss of power, and be rough accelerating, unless
when going, you ease off the accelerator, where it smooths down, and
if you gently ease the accelerator down, it runs fairly smooth. goes
back to lumpy if its pressed down harder. It also seems to be a lot
smoother at higher engine revs.

Happened only about 2 days ago, during a run down a country road,
stopped at a railroad corssing, and it was suddenly like this.

hope someone can give me some hints
thx.