hmmm
hi there.i used to have a mk1 1300 i think it was.anyway it used to do something slightly similar.the problem with it was that it had been run on cheap oil and what was happening was that there,s an oil induction pipe which sucks oil from inside ure sump.the pipe has a wire gauze like material over it,which in my case was clogged up,which let some oil through at the start but then as i drove it a little sucked the gauze up against the bottom of this pipe.i wonder if ures is the same and maybe as the oil heats up a little it may be thinning and letting the oil up through.best bet is to try a different oil switch first,failing that screw the sump off and have a look at the gauze on the oil induction pipe in ure sump
let me know how u get on there