What was found
JUST AFTER LEVEL OFF AT FL 400 AND FUEL CHECKED BALANCED AND NORMAL PLANNED BURN, RIGHT TANK SHOWED 1000 + IMBALANCE. MOMENTARILY OPENED CROSSFEED AND TURNED OFF #2 FUEL PUMPS WITH CROSSFEED OPEN AND ONLY #1 PUMPS RUNNING, #1 TANK QTY DECREASED RAPIDLY. RAN QRH AND RECONFIGURED PUMPS. FUEL IMBALANCE AND OTHER INDICATIONS RETURNED TO NORMAL. GROUNDED - YES. ACCOMPLISHED FUEL COMPARISON CHECK PER B737-800 AMM 28-41-00 AND CROSSFEED VALVE OPERATIONAL TEST PER AMM 28-22-00.