After many weeks of testing and testing, VC seems to have isolated this problem.
I shall leave it up to V3G to explain the technical details, but basically it has to do with the drivers for the PCMCIA cards and, in my case, for my handset's modem.
I would like to thank V3G as well as all the guys from VC and NSN who were involved (I don't think it will be appropriate for me to name them by name here) in solving this irritating problem! I have never ever experienced such fantastic service from an ISP before!
You guys are the best!!
Many thanks!
Steinie