My M20 won't rev past 5000 rpm - tragic

[fecklessone]

Hi Guys,
I've a 1988 320i cabby that runs perfectly until it reaches about 5000rpm in any gear - when it feels like it hits the rev limiter and won't rev any higher - just when things are getting interesting.
The problem is independent of throttle position or engine load. Don't think it's fuel supply.
I've tried a different ECU (from same model) - no difference.
When I first pulled off distributor cap and rotor, cleaned & re-assembled, it revved up to 5500 a few times before settling back to where it was before. Strange...
After that I replaced dizzy rotor, ignition leads, dizzy cap; checked and cleaned all connectors. Still no joy.
Plugs look OK.

Could it be Crank Position Sensor? I've pulled it off & given it a clean but not replaced it yet.
Feels like an ignition issue more than worn camshaft or something (engine feels strong & only done 110,000)

Anyone solved/had this problem?
Ideas gratefully received. Thanks

[Speedtouch]

I had a 1984 B-reg L-Jet 320i, which ran fine until I took it Halfords for a new cambelt and it did the same thing afterwards; I reckon they fitted the belt one tooth out.

[fecklessone]

Hmm...interesting. Mine just had new cam belt fitted too - not Halfrauds though - someone I trust, so should be OK. Wouldn't it run pretty badly through the rev range if cam timing was out?

[Speedtouch]

Mine ran fine, until you got to about 5,000rpm ish, when it just wouldn't go any higher.

[fecklessone]

Did you get it fixed?

[Speedtouch]

No, in the end I just put up with it for a few months then sold it.