3d Extruder temperature lower automatically (Prusi I3) when I command to start print from SD card; extruder temperature raise slowly and print start but with in 4-5 minutes suddenly Extruder temperature lower automatically.
Please consider to reference your precise model (genuine Prusa i3 or clone?) as well as the slicer software you are using.

You should be able to note from the panel information what layer the change appears. For example, if you are printing 0.200 mm layer thickness and the temperature change begins at 10.200 height, you have an important piece of information.

Open your gcode file in a text editor and look for the Z reference in the 10.000 or 10.200 range, along with the temperature gcode command. According to the reprap wiki,, the gcode command to set extruder temperature is M104 followed by Sxxx where xxx = temperature in °C. For example, you should see something like this near the beginning of the gcode

M104 S205; set extruder temperature to 205°C (comments mine, some slicers also include comments)

but if your gcode is faulty, later in the body of the gcode you may see a similar command:

M104 S000;

the number after the S could be anything. Consider to examine any M10x codes as there are different results for various codes. 

If you find you have a dropped extruder temperature due to the gcode, double check your slicer settings to see if a second process has been initiated.

I have a dual extruder printer and will usually keep the second extruder cold until just a few layers prior to printing supports at a level above the bed, then turn it on to do the supports, then back off for the rest of the print.