'98 S-10 4.3 surging at low rpm

'98 S-10, 4.3, auto, 4X4 131,000, has a "surge" between 1500 and 1900, and then again between 2200 and 2500 RPM. Only seems to be present when
warm, when engine is not under load or coasting. Slight increase or decrease in throttle position seems to help the situation. Range of surge is 200 - 300 RMP, cyclic, about 1 cycle per second, slowly dampens after about 10 - 15 cycles (possible change in engine load?). There is also a noticeable "buzz" in the same ranges.
So far the distributor cap and rotor have been replaced, fuel filter replaced, throttle body cleaned. The battery terminals have been cleaned, the engine ground has been cleaned, the positive lead at the master fuse and relay box has been cleaned. The surge still remains. OBDII shows no codes. I'm looking for suggestions as to where to look next.
Thanks in advance,
Steve Sharp snipped-for-privacy@comcast.net
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/I had a siilar problem. Turned out to be bad vacuum leaks. Check for those, and maybe your MAP sensor.

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burntkat IS AT comcast.net wrote:

Traced the lines, nothing noted. Couldn't get to the xmsn vacuum lines however, will check them this weekend. Do you remember which line(s) were giving you the problem?
Steve Sharp snipped-for-privacy@comcast.net
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Check them all, and more than just a visual check. You're not working on my truck, so what was wrong with my truck won't be the same thing.
Bear in mind there are a lot of vacuum fittings behind the heads up against the firewall. Maybe it's back there?

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