I have the same issue, up until 1.26 I had no issues and could play multiplayer for hours, since this patch I go from 50fps to about 4 fps on or about the hour of playing mark making the game impossible to play. Stop multiplayer and restart and fps is good again until the hour comes up...
Please find a resolve for this issue as it is making the game unplayable in multiplayer...
My specs:
MoBo: AsRock 990FX Extreme
CPU: AMD 4170 (4.2GHz Quad)
RAM: 8Gb Corsair DDR3 1600MHz
GPU: AMD 280X Toxic (3Gb)
Game installed on Samsung 1Tb HDD 64Mb Cache
Game settings are all on High but have reduced them to Med but with the same issue, this is not system related but software as everything was good before the last big patch.