turbo problem

im working on a 89 toyota surf 2.4 turbo. when it is cold the turbo seems to be working find, the light on the dashboard comes on and you can feel the turbo working. After aprox. 10-15 mins of driving the turbo light stops working and so does the turbo, so it feels like it. i have had the turbo over hauld and and all the intake pipes changed, but that has not fixed the problem. can any body help me?, and does anyone have any wiring and vacum diagrams.