Using kdenlive 17.04.3 - much better performance on my pentium (r) 3.3x2ghz 4gb with NVA8 gfx.
Everything was fine. then yesterday i was getting no picture on either preview or project monitors. instead a kind of color-geometric mess whenever I move the mouse over the monitor area. Will try reinstalling but any ideas anyone?

You are on Linux? If yes it could be that you get an update on MLT, QT or another part of Kdenlive from Linux itself and the «chain» is broken. Try to install the AppImage.