The best way to be sure the starter has a proper ground (and the way it used to be done before truck builders started trying to cut corners by using less copper cable) is to run a 0 gauge cable directly from the ground side of the batteries to the ground post on the starter.
This would be in addition to the existing ground cables attached to the frame.
No matter how good of a connection there is where the ground cables bolt to the frame, there is bound to be some amount of resistance, and that resistance will only increase with time due to corrosion from salt spray etc.
The newer Internationals where I work have the ground cable to frame, then ground cable from frame to starter setup, and we have had to unbolt & clean the connections on those too, but we went a step further and ran a direct ground to the starter as previously described.
Also, in a previous thread I described another cause of poor starter performance:
http://www.classadrivers.com/forum/t...art-truck.html
Quote:
Also double check those short cables that connect the 3 or 4 batteries together.
We've had a couple of those where the copper terminal fractured inside the plastic covering at the cable end. If this happens to one of the cables between bat # 1 & 2 in the lineup, it causes the truck to only be able to utilize one battery out of the 3 or 4, causing sluggish starting. Externally, nothing appears wrong, but you need to pull them all off the batteries and check the cables for continuity. Also visually inspect the terminal ends, if they look questionable, replace them.