Power/Acceleration problem when driving

Hard to explain but will do my best.
Whilst driving, the car loses acceleration but the revs remain as normal and I can still change gear which lowers the revs but still have no acceleration but then acceleration will return.
Also, when car is first started and for the first 5mins of driving the idle speed is around 15000rpm
Any ideas why this is happening?
It's a Getz GSI 1.1 manual

 

15000 rpm (15 thousand rpm) is a very high rpm
And for most engine, would burn them up anywhere between 5 to 8 thousand rpm .. lol

When engine is cold, the idle at 15 hundred rpm is not unusual.
After warming up it should drop to around 5 to 7 hundred
That would be normal.

With the lack of more information and testing, would be guessing
Big wild guess would be the clutch is slipping
Probably wrong.

Better to have a professional mechanic look at it.