I run the larger dry cell that battery direct sell..
http://www.batterydirect.co.nz/vertex_drycell_battery.htm
Seems to work perfect in my car. Cranked it for ages when i was havi g an issue and it never even looked like goin flat...
You sure your not having other issues?
Yup sounds like a dying shock.
Get them rebuilt.
Get hold of Chris at "suspension tech" in Auckland.
Www.suspensiontech.co.nz
He is the man when it comes to rebuilding bilstien shocks.