Photovoltaic conundrum

EEGG

This is a 6 kWh photovoltaic system with 24 X 250w 60 cell Yingli panels, connected 8 in three string to a central inverter Fronius IG 5100. The inverter is connected to the grid 240/120 V split phase, grounded.

The panels have a cyclic power production. When they reach 5.9 amps and 232v they drop quickly to 1.5/2.0 amps and 242v and climb slowly. Few days the system is fine. 

I disconnect each string without any effect. The ground are fine. Connections are fine. No volts between Neutro and Ground. No visible archs or defects on the panels. Polarity is fine. The problem is not present without load in the panels but this observation is only in a short period of time. The inverter is fine, I exchange it with a new one and the problem persist.

I will appreciate any assistance

Regards.

Enrique