Running problem

[Kentish]

So I first got this problem when I was using the bike in heavy rain. Thought I had solved it but havn't

Basically after around 40mins of riding the idle starts going rough, the bike then pulls away very "jittery"

It's hesitant to pull away low down. The bike runs fine when you are further up in the rev range....

Any ideas? I have changed the plugs and checked TPS