I completely agree with checking the belt routing. OP states in the first post that the belt is brand spanking new, so there is a strong possibility of it being routed incorrectly or being the wrong size, if the problem presented immediately after the belt change.

Routed incorrectly, the belt WILL appear to have proper tension on it, but it WILL slip. BTDT, spent hours cleaning my "brand spanking new" belt and cleaning pulleys and scratching my head, until I finally dug out the diagram.

However, if the problem was there before, then looking at the ball joints, power steering fluid viscosity, and any other issues like worn struts is a good start.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC