Dyno power runs

Hi i have a query i just bought my 1st dyno and am getting myself familiarised with the machine before moving on, atm ive just been power running my van and familys cars what am finding is the 1st power run i see the expected hp and as i continue more runs the power is dropping and dropping i have cooling fans etc so am doubtful its too hot but could it be? The cars have been diesel any advise would be great

hello, I would almost guarantee that it is the air intake temperature being affected by heat sink

So quick? basically 1 run after another ? So the ecu is pulling power, fuel, timing as temps increase ? So i need a huge fan?