do a voltage drop test on the starter cable
hook up 1 lead of the meter to the B+ terminal of the battery
hook up the other to the large cable terminal on the starter (the threaded post with alligator clips, not the cable terminal ring)

if the cable is good the voltage while cranking will be around 0v
if the cable is bad it will read voltage (even just 1 or 2 volts is bad and would indicate resistance in the cable where there should be none)

repeat this test for the negative cable (B- and the engine block or the metal housing of the starter)

and the Ignition timing? and try a new battery, your old batery may be revesing polarity and still drawing down the boost, rather than the boost going to the starter...

You say if turns over but very slow even while being boosted.
Have you checked out the starter???