Check the filter head, if the diaphragmn is on the way out it allows air to get in which could cause all you have, they suffer terribly from this, a lot of peeps carry a spare one, 10 minute job to change only need 2-3 spanners, they are about £12-£15 for a new one.
Another check you could do is pump the primer prior to starting 1st thing in the morning, if it's not solid to start with but takes pushes to make firm then this again suggests the filter head(diaphragm).
tabetha