Baxi 105e CH pressure problems?

Baxi 105e has 2 issues...regular but intermittent:

1. When CH not on, pressure can drop to 0bar over fairly short time (ie between being on in evening to low pressure warning by next morning - approx 7 hours). This is intermitent

2. When CH is on pressure can rise to 3+ bar and the PRV starts to dribble. Again, this is intermittent (although more regular than the first issue).

Any ideas where to start?