2001 206 Diesel LX engine rattle/ECU Issue??

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Hey all, I have a 2001 Diesel Peugeot 206 that I've had for about 6 years now, the problem I'll explain shortly has been ongoing (on and off) for the past 2 years.

Now the problem revolves around the fact that I'll be driving along happily and then the engine starts rattling quite bad, there are no dash warning lights and there is a significant loss of power. The first time it happened I pulled over, lifted the bonnet while it was idling to have a look if it was anything obvious (which it wasn't) and then turned off the engine and ignition to have a look. I couldn't see anything so five minutes later I started it up again and it was running fine! It ran fine for nearly a year ... only for the same thing to happen again twice in the space of a month. Now the consistent things seem to be on all occasions it happens I'm travelling a constant 50 or 60mph and the problem seems to be electronically related as in I can switch off and restart the engine but the rattle will still be there, but if I switch off the ignition and wait for the "click" that happens after a few minutes when the dash "powers off" (the speedo and rev counter reset) and you can no longer power the electric windows without turning the ignition on and THEN restart the engine, it runs again fine!!!!
It seems to be like "rebooting" the car! Does the ECU reset itself when powered off?? is it likely to be a faulty sensor somewhere that is triggering the ECU to change the timing or some such thing, but when I "reboot" the car everything is reset and it's fine?? It's really odd .... I begrudge paying a garage to take a look for what it might cost me seeing as this seems to intermittent ...