The student pilot on her second solo cross country experienced a partial loss of engine power during the initial climb. She made three attempts to land on the takeoff runway, each time flying a left traffic pattern that was lower than the preceding pattern. Ground witnesses saw the aircraft fly patterns that were 'low, close and with tight base to final turns.' On each base to final turn, the aircraft overshot the turn due to a strong left cross wind. During the third base to final turn, the witnesses saw the aircraft enter a very steep bank, stall, and hit the ground on the left wing tip. A post crash fire destroyed the aircraft. No evidence of a catastrophic engine failure was found. Four of the spark plugs failed a pressure chamber test, with three of them showing gaps in excess of .022' (normal gap is .016) and were very worn. Review of the maintenance records showed that the maintenance facility only replaced spark plugs on a one for one basis instead of in complete sets.