I metered battery V whilst starter cranked by jumping at relay. 10.5 V, and fan belt barely moves. Then as if something needs heat to make good connection the starter droans faster and faster. It reminds me of the way a really advanced distributor will make for a hard start.
Because I like you guys, I 'm gonna confess to you that the starter is hanging from 2 ex BMW 10x50 mm / 8mm socket head 1/2 shaft bolts. Also the suspect positive cable is a re-purposed BMW 1cm cable for the same use. The battery, likewise is a backward posted international battery from a 528e. It is 2 yrs old and I keep it up with occasional charges from a 1/2 A HF 4.99 special. The cable fits like it was made for the job and sure acts OK, but it is the only constant in 2 starters behaving the same way. Maybe it is the battery, a Duralast??