1998 Oldsmobile Achieva eng surge

At warm idle car surges = lopes elect system shuts down each time it surges & comes back it is like If you disconnect batt & rehook it back up intake vacuum is 14 to 15 study at idle & raises going above idle rpms as high as 17 to 18 & settles back to 14 to 15 ckd cat temps about same temp in & out rt some times slightly hotter car runs ok on road mostly have renewed fuel tank & fuel pump filter, & all purge parts, computer, all sensors & solnoids complete tune up, run sea foam through intake oil looked black afterwards chged it car had over a gallon o water in tank & was run about a 100 mi before changing fuel tank & pump & filter & all other parts it was a lot o carbon & water burning in engine could this have overheated o partualy plugged cat. Thank you