Loss of Half Power? Car turned off and on and problem gone.

This has happened twice now about 2 months apart. Sitting at lights in neutral, take off in first giving full accelerator and slowly takes off, get to 5th gear after a long 5 mins of slowly accelerating with maximum throttle, with a maximum speed of 80kmph, will not go faster. Does not over rev or stutter, its simply got minimum power and a slight burning smell. After pulling car over and turning off, then straight back on again, full power is returned. Clutch or electrical im not sure?