A surge sounds like too much timing,or lean.air pocket in the cooling system??Fan should be about 3/4-1 in away from radiator.Put a sheet of paper on the front side of rad,(grill side) see if the fan pulls it into the rad.
remove t-stat and pull the center out(spring and plunger) and just run it as a restrictor plate. will flow fine,no chance for air pocket.Did you add a stall convertor?? might need an extra trans cooler.Any possibility of a vacuum leak??all ports sealed off.pcv valve, brake booster.
Usually hotter- higher rpm and then idle cools down is a flow problem.plugged radiator.Might be fan distance in you're case.