THE OWNER HAD BEEN CONDUCTING HI-SPEED TAXI PRACTICE WITH SHORT TAKEOFF/LANDING HOPS IN THE SAME DIRECTION BUT WAS UNABLE TO OBTAIN LIFT-OFF. HE THEN ASKED THE PLT TO SEE WHAT THE PROBLEM WAS THE PLT SAID HE WOULD NOT ATTEMPT TO FLY THE ULTRALIGHT. ON THE 2ND RUN THE ACFT BECAME AIRBORNE BY ABOUT 10-12 FT AGL. POWER WAS REDUCED & THE ACFT NOSED IN HARD FOLLOWED BY A BOUNCE INTO THE AIR NOSE HIGH. POWER WAS APPLIED & THE ACFT CONTINUED A CLIMB JUST CLEARING POWER LINES. THE ACFT THEN LOST ALTITUDE, WOBBLED SLIGHTLY, & MADE A SLOW RIGHT BANK GAINING ALTITUDE. DURING AN APPARENT ATTEMPT TO RETURN TO THE FIELD IT WAS OBSERVED TO SUDDENLY DIVE TOWARD THE GROUND. THE PLT HAD COMPLETED A CHECK RIDE IN A SINGLE-SEAT CGS HAWK DURING THE SUMMER OF 1982. THE OBSERVER REPORTEDLY STATED THAT THE PLT WAS NOT AS SMOOTH AS OTHER PILOTS THAT HAD COMPLETED SIMILAR CHECK RIDES.