72E picks when it starts when icy cold

Hi, I've got a 72 911E that all of a sudden (since I dragged it up to northern Indiana from Fla.) decided that it picks when it will let me start it. I did the lights on start test and they don't go out but do go very dim. The car was run at most twice a month for 6-7 years, so a lot of sitting took place. I don't think it is the battery, I suspect the wiring or solinoid.
Am I right in thinking I should have a 1.5 hp starter (9112200060) I recall this same issue back when I purchased the car in 87. I was active dutywhile my father did the wrenching and he claimed the car had a VW starter on it. Could he have been mistaken and it only had the .8 hp starter. He said he replaced it with a "bigger" one I can only assume the 1.5hp until i crawl under there, and it's freaken cold up here so I'd like to put it off for a while. Anyway it was suggested to run a heavier gage cable from the battery to the starter. It seems my cars timeframe of build has put me in a spot where I need to do some detective work and figure out just what is under there.

I think I know what I need and what to do, but I figured I'd draw on all your experiences and see if I am forgetting anything. The starting difficulty occurs when temps are around 15-20 F. It was warmer today (40F) and no trouble.

What do you guys think?
Thansk, Dennis
72 911E
91 5.0
69 spit

I noticed your 72E was a coupe, any chance it is a sunroof? As for the starting, sounds like a weak system indeed, before starting next time check the voltage at both of the batteries, they have a tendency to go bad after in some cases as little as five years. Then start by checking the battery grounds and the connection to the starter itself, the distance between the batteries and the starter is longer than on a conventional vehicle therefore the resistance is greater and the voltage is lower on the long cable run to the back. If all of these appear good I would first give the batteries a full charge on a charger that will distplay that they are indeed at full capacity and then put them in the car and try to start it on one of these troublesome cold days. If none of these seem to fix the problem then the starter seems to be the likely suspect.