'92 Cabby - Burning Smell

I've got a '92 Cabby & I have been having problems lately with a burning smell.  It first occurred shortly after we had all the snow.  I took her out after being sat parked up for 2 weeks & she very quickly began to over heat.  A mate had a look at her & one of the pipes had iced up.  Once that had cleared she seemed fine.  Now every time I take her out I seem to get a burning smell when either sat in traffic or going about town at speeds 30mph or lower.  Has anyone else ever experienced this or have any ideas on what the problem may be?  Thanks.