My old 33 had this problem and initially I thought my foot was touching the clutch (due to the pedals being so tight). However, it was found out to be a 'soft-metal' bush in the gear box had worn down causing play within the gear itself.
Unfortunately I didn't find this out, until the whole gearbox packed in as a result ... but the gear slip was the early sign!
I'd personally check it out and see if this is the problem and solve it before it takes your car of the road and lands you with a hefty bill. However, hopefully some more experienced 33 owners can perhaps give other possible reasons.