Poor running KV

[audicoupeparts]

KV in a b3 90 quattro
Bought as a project.
Had poor idle.
Changed ISV rubber pipe for a hole free one.
Idles fine now.
New dizzy cap and rotor arm
New plugs
Small vac pipes replaced.
Starts and revs fine for about a minute
Then gradually starts to spit its dummy and starts missing when revved
Won't then Rev over 4k, then gradually gets worse and won't Rev at all and cuts out.
Any ideas folks?
Been a long time since I've messed with a KV so running out of ideas.