ofdm signal (dab) decoding problem

hi,
i am working on the development of an ofdm receiver (dab receiver). After
frequency synchronisation, when i differentially decode the ofdm symbols,
i get 5th symbol corrupted i.e. the constellation of 5th symbol is not
centerd on the multiples of pi/4. When i shift the fft window for 5th
symbol 7 samples backward, the contellation improves, but it is still very
noisy and not centerd at the multiples of pi/4. This problem is repetitive
for every 5th symbol.
I think this is due to the sampling frequency offset.If anybody have idea
of sampling frequency removal in ofdm systems,please help me.