1988 Thunderbird Turbo Coupe

1988 tunderbird turbo coupe
170,000 miles, 2.3L Turbo, 5 speed

hey guys. I bought this turbo coupe from a guy saying it only needed a new battery. Got it home, installed a NEW battery and still have the same problem. Here are the symptoms, I hope someone can clue me in on a fix:

turn key over to start the car. Engine barely turns over, most times not enough to start but sometimes I get lucky and it fires (acts like its not getting enough juice from the battery). Tried hooking jumper cables up from a running truck but they didnt help the cranking speed. Added an extra heavy duty ground from the battery to the frame and from the engine block to the frame, didnt help. Replaced the solenoid with a new one, same problem. Checked the wire from the solenoid to the starter and it looks fine and tight on both ends. Both wires from the battery are in good shape as are terminals. The only thing I can guess is maybe the starter is on its way out. If anyone can give me any ideas you would be a hero! Thanks alot! Cory