Some say that a single generic "600w" psu just isn't enough to drive two r9 270's. If that 600 watt psu is from a high quality manufacturer like Corsair, Seasonic or Enermax, things should be theoretically fine. Otherwise, a 650-700w psu is needed. So the bottom line is, to avoid catastrophic failure, the gpu's turn down their clocks to consume less power so such an event wouldn't occur. A thing you could try is, disconnect one gpu entirely (for less power consumption) and see if it gives you stable framerates. If the issue still occurs, the problem lies somewhere else.
Intel I7 3770K @ 4.5ghz, Cooler Master Hyper 212 Evo, Gigabyte Z77X-UD3H, Gigabyte HD7970 ghz edition, 4x4gb Kingston HyperX Beast 2133mhz, Seasonic Platinum 860, Samsung 840 Pro 256gb, OCZ Vertex 4 128gb, WD Black 1TB, WD Green 3TB+1.5TB, ASUS Xonar Essence ST, ASUS VE278Q, Windows 3.11