78 300SD, my car.

Lately the car has been really hard to start. Cranks forever but starts eventually.
I was hooking the battery up to a charger (hard starts are bad for the starter and battery) and found the vacuum line to the top of the IP was off. I replaced it and it seemed to start quick and fine. Maybe it had been off for a while and I didn't know it. What does that line to the IP do?

Also while the car is cranking, the oil pressure gauge will go to 2 and tap it's way up to 3. As soon as the needle gets to 3 the car starts.
It only starts if you let it crank really long, I know very bad for the starter. But if you give the starter time to cool, then the needle on the oil pressure loses it momentum. It goes back to 2 and I have to start over again.
There is no difference in how fast the car cranks as the needle moves. It just suddenly starts. After it starts the car will start again easily for several hours. 4 hours of sitting though and the car will be hard starting again.
This morning the car had a full battery charge (on charger all night) and it was hard to start.
Any ideas as to why?

On a side note, the gresecar was doing the exact same thing. Not driven it though since the keys got misplaced.

Thanks guys. Sorry for my absence, life is overwhelming right now.

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Ginny in Denver-ish
78 300SD, 265K (mine)