If it always occurs at the same throttle position (rather than the same rpm as you originally stated), then perhaps it's reasonable to suspect the TPS. Testing it, as suggested by N3LYT, is pretty easy if you have an analog ohmmeter, though I'm unsure about his statement that it should be linear; but in any case it should be smooth and monotonic.
As for the Haynes manual, in my opinion it ain't a patch on the factory service manual. The Haynes manual usually covers multiple years and models and is much smaller. The factory service manual is for a specific year and model and typically is two or three rather thick volumes. No comparison.
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-- Carl
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