engine surging

My 1995 G20 van surges between 55 and 70mph. I have changed the throotle body fuel sensor, the map sensor, the tp sensor, the wires and plugs, cap and
rotor, EGR valve and EGR modulator, new fuel cap, O2 sensor, burped the cooling system, and now I just don't know what to do. Van has 172000 miles and I've been fighting this problem for six months(32000 miles). The engine temp is running good and the charging sysem is fine. I think it has something to do with the mixture but I don't know what else to do.........
Dave
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I had a 1995 G20 do the same thing, finally took it to a shop. A new pickup coil and distributor shaft fixed the problem.
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I just replaced that yesterday. The engine lite still came on and the surging is almost gone. I dialed in the distributor timing by feel......... maybe I need to have it hooked up to a diagnostic unit to be dialed in...... what do you think?
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