Bought an original 89 GT, only 20K miles. Off trailer it started and ran great for a couple weeks. Fired first turn no lag at all. Left lights on once and had to use a jump box to start, ever sense then, the battery drains if sits for a day or so, it lags and very sluggish on start. Put on a new alternator, same issues. Battery tests fine... I'm not a major mechanic per say but would like to know what to look for or how to go about correcting things so I can enjoy it. Please advise.