power comes and goes

1.6hdi
power comes and goes, it will drive fine for a while with enough to give then it just seems really flat and sluggish

no smoke that I can notice

when it goes flat it does seem to use a lot of fuel, I think the engine note changes too

I have plugged it into my basic code reader, theres no codes showing.


also every now and then the speedo stops working, the power seems to go flat when that happens, sometimes a whack of the instrument panel brings it back, sometimes not and I have to switch off, then switch back on and it clears.


any help would be really appreciated

thanks