MPV surges when hot

My 98 MPV 4x4 (160K) All-Sport surges badly while accelerating when hot - either after running a couple of hours in cool weather, or much faster when over 25oC. In the worst case, it becomes undrivable if more than the slightest acceleration is required (eg up hills!). If the vehicle is stopped for a few minutes, the problem temporarily disappears. The distributor was replaced by the Mazda dealer; it has had new fuel and air filters and been given a "super flush" of all lines and fuel injection assembly. I suspect either a fuel or air supply issue related to faulty control (thermostat, computer? Fuel pump?) But no errors are generated by the onboard computer and of course the problem is impossible to generate for the dealer who has been unable to suggest a cause! The water pump and fan clutch was replaced soon after we purchased the vehicle; I don't know if the problem was present before then, but the Mazda dealer denies it (we wonder if they damaged something during installation). ANY SUGGESTIONS PLEASE?