Weird charging issue

Ok on my 89 it has the 3G conversion,and battery is in the trunk.I dont drive it often.So i have noticed that when i drive it the gauge reads fine (1/2 way) but then sometimes when i go to start it it cranks over very slow almost to where it barly barly starts, like the rrr......rrr.....rrr then it fires.The gauge reads good still and alls fine.This usually happens after i drive it.If it sits for a few hours or more it starts just fine.Any ideas?It is a new battery but since it sits for periods at a time between driving it could the battery just be dead?But then that dosent make sence cause if it sits it usually starts fine!Just when i do drive it it cranks over very slowly!BUT also like i said the gauge reads fine,even though it cranks very slowly.........Now another question diffrent stang.It's my DD 4 banger.I washed under the hood yesturday,well now the gauge reads low volts.The lights are as normal,it starts as normal everything electrical is as normal.Plus the stock alt is less then a year old!It has never done this untill i washed under the hood.So could it be just water got in somewhere?I'm gonna give it a few days and see how it goes.peace









john