1971 MGB Reverse slipping when warm

I've had my MGB for over 10 years and have not really had any problems with it of note. Everything works. However, in the last couple of months I've had a re-occurring problem with my reverse gear.
When cold/warmed up I can back it out of the garage just fine with no hint of a problem.
When I run it hard for a while, such as when I do freeway driving, the reverse goes out. The clutch engages but all I can is a spinning sound and the car will only moves backward at most a foot.
I'm worried that it's something serious, as all the fluid levels were okay when I had it in for normal servicing.

Thanks

How old is the clutch?

Not exactly sure, It's not the original as the previous owner was an import car mechanic and had replaced both the engine and the clutch. But I don't know the exact mileage that was on it when I purchased it. I estimate that it's less than 40k as I put less than 2000 miles on it a year and he had it for two after he replaced the engine/clutch.

Does the symptoms sound like a worn out clutch? I don't have any problem with any other gears and as I said when starting out reverse works just fine.

Oh well, if a new clutch is needed I put it in myself over the winter. Then taking it to a body shop for a sanding and a new paint job.