Usually these types of problems are caused by poor electrical connections, undersized wire or damaged cables. Clean every electrical connection between the battery and the starter (including grounds). Make sure that the cables between the battery and solenoid and the solenoid and starter are not damaged and that they are the proper size. If neither of those fixes the issue, measure the voltage drop between the battery and solenoid, across the solenoid and between the solenoid and starter to find out where the most voltage is being lost. Replace the components that are causing the largest loss.