e300td probs p1403 fault code

[tonka12]

hello all ive recently purchased a 98 e300 td estate its done 231k but has a very good history so didnt see many problems.it drives very well [when not in limp mode] so a few days after buying it it went into limp .plugged it in using the launch at work and got p1403.i started by taking off the inlet manifold to clean out the egr but it wasnt as bad as some ive seen so gave it a quick fettle and all was well. went into limp today same code so changed the vacuum convertor and all was well untill half way home from work and back into limp again. so please tell me if im looking at the right things its doing my head in now cheers in advance russ