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I was looking through my scans yesterday and came across a TSB on engine surging when driving....... I have seen several posts re this problem recently. The TSB basically reads the Engine temp switch could be the problem as it will inadvertently send power to the Timing advance solenoid which will cause the timing to change thus surging. Fix is replace the Engine temp switch....
How would you know...... I would make a long enough wire with a bulb attached, hook it to the solenoid connector and the spade terminal..... make the bulb visible with the hood closed while driving....... if you get surging and a lit bulb then ....... Bingo.