1971 MGB Reverse slipping when warm

[Rips71MGB]

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

[KimMG]

How old is the clutch?

[Rips71MGB]

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.