TRAINING FLT ENCOUNTERS FREQ PROB TRYING TO MAKE CONTACT WITH TRACON FOR PASSAGE THROUGH CLASS C AIRSPACE.

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ENRTE FROM COLUMBIA; SC; TO FLORENCE; SC; 3500 FT IN A C172 WITH 2 RADIOS. LEAVING CAE AIRSPACE WE SQUAWKED 1200. WHEN COMING CLOSE TO SSC AIRSPACE WE TRIED TO CONTACT SSC APCH ON FREQ 125.4 AND 118.85 (BOTH FREQS ON BOTH RADIOS). UNABLE TO ESTABLISH RADIO CONTACT WE STARTED CLBING TO GO ABOVE. (WE DID NOT HAVE ANY PROBS WITH THE RADIOS ANYWHERE ELSE.) APPROX ABEAM THE ARPT THE CTLR BECOMES AUDIBLE ON 125.4. EVALUATING THE SIT AFTERWARDS; WE REMEMBERED THAT WHEN PASSING THE SAME AREA ON AN E TO W HDG AT 4500 FT ABOUT 1 HR EARLIER WITH RADIO CONTACT; THE FREQ BECAME SUSPICIOUSLY QUIET AFTER PASSING THE ARPT. THIS RAISES THE QUESTION IF THERE WAS SOMETHING BLOCKING THIS SIGNAL ON THE W SIDE OF THE ARPT AT THIS TIME. I HAVE FLOWN IN THIS AREA BEFORE AND HAVE NOT HAD ANY SIMILAR PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THIS IS NOT A PROB MOST OF THE TIME. HE HAS DISCUSSED THIS WITH SEVERAL OTHER FLT INSTRUCTORS WHO HAVE EXPERIENCED THE PROB ON OCCASION. IT IS SUCH AN INTERMITTENT PROB THAT HE DID NOT NOTICE UNTIL HE WAS CLOSE TO THE CLASS C AIRSPACE AND CLBED ABOVE TO AVOID IT. THE FLT INSTRUCTORS AND STUDENTS HAVE ALL BEEN MADE AWARE OF THE FREQ PROB SO HOPEFULLY NO ONE ELSE WILL GET CAUGHT UNAWARE.