Strange display problem, frame rates drop intermittently

Have run into a graphics problem using P3D v4.2, I run four screens so for me frame rates are low, if I can get 30fps by turning scenery etc down I am flying well. However I find that every sixty seconds or so the frame rates drop to single figures and the suddenly picks up again. Have tried turning off Windows updates and anti virus. Whilst scenery can hit rates I don't think that is cause, I get the problem in the cruise as well as in heavy scenery. Nothing odd showing up in task manager.
PC is i5 OC to 4.4ghz. Dual GTX 970 graphics cars. Windows 10 64 bit.
Never used to do this with earlier versions of P3D.
Barry
www.a320sim.com