trouble cold starting 90 Jetta

I recently got a '90 Jetta. When I picked it up it had been sitting in the snow for a few weeks, but started right up no problem. Recently, it will crank and crank and sometimes catch. Other times, it dies all the way down and I have to jump or charge it back up. My daily commute isn't enough to recuarge it fully, but I've trickle charged it overnight and after a few days it still eventually quit. If the car sits out all day in the cold I have a harder time with it than if I come back and try to start it after only a few hours. Any ideas? Thanks!