I lot of times those electric fans don't work well because they don't use a shroud and only cool the area the fan is actually over, near nothing compared to the size of the rad.
If your burb doens't over heat doing done the highway, but only in slower traffic it HAS to be the fans and or air routing, period. At speed the fans and or shrouds are doing NOTHING. So they are not the problem when it over heats at speed (thirty five or so and higher). At slow traffic speed or stop and go traffic, the radiator is relying TOTALLY on the air being forced thru it by the fans.
So, that being so, your fans need some sort of shrouding (or toss them and go back to a good old fan on the motor).
Think about what a shroud does. It FORCES all the air being sucked or blown by the fan THRU the the entire area the shroud covers. SO if you make a shroud that covers the entire radiator your fans are then cooling the whole thing instead of just the area the fans are hitting now.
