DRG AN EN ROUTE CLIMB, AILERON CONTROL (CTL) BCM PROGRESSIVELY MORE STIFF. AT CRUISE ALT, THE AUTOPLT DISENGAGED & THE AILERONS WOULD NOT MOVE. AILERON CTL WAS RESTORED AFTER A DSCNT BELOW THE FREEZING LEVEL. THE ACFT WAS FLOWN BELOW THE FREEZING LVL FOR THE REMAINDER OF THE FLT. A POST-FLT MAINT INSPN REVEALED NO PHYSICAL EVIDENCE FOR THE TEMPORARY LOSS OF AILERON CTL. AN INVESTIGATION REVEALED THAT BEFORE TAKEOFF, THE ACFT WAS EXPOSED TO RAIN FOR ABOUT 50 MIN WHILE PARKED AT A GATE. THE TEMPERATURE IN WASHINGTON WAS IN THE MID 40'S. THE OPERATOR SUSPECTED ICE BUILD-UP ON THE AILERON CABLES. THE MANUFACTURER HAD ISSUED A SERVICE BULLETIN (SB) (SB B-757-0079) TO REMOVE THE AILERON CABLE GROMMETS & APPLYA SEALANT TO ELIMINATE THE PROBLEM. THE SB HAD BEEN COMPLIED WITH. AFTER THE INCIDENT, A REVISED SB, DATED 11/6/86, WAS ISSUED TO REPLACE THE AILERON CABLE GROMMETS WITH NEW GROMMETS WITH INCREASED INTERNAL DIAMETER. ALSO, AN OPNS MANUAL BULLETIN (NUMBER 87-1) WAS ISSUED ON 1/22/87 TO ADVISE FLT CREWS.