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Hi Len
Your hydrauic downfeed has no effect on the derailing of the blade, that's from some other fault. Hydraulic downfeed gives a much more stable cutting action which makes the blade is less likely to derail if anything.
In order to identify the problem you first have to know from which wheel (upper or lower ) that the blade derails from.? To do this remove the blade cover door from the sawframe (unscrew the two hinges) and turn the saw on and watch the blade.? Its designed to run so that the back of the blade just about touches (sometimes hits it and sometimes not) the rims of both wheels.
If its running so the back is clear of the top rim, then the tilt of the top wheel needs adjusting so the blade tracks properly (Slacken blade tension, then loosen the bolt that holds the top wheels axle, and screw the grubscrew below it in ~1/8th of a turn at a time, then tighten/retension and recheck tracking)
If its clear of the bottom rim but OK on the top, then its likely the outer bearing on the gearbox driven shaft has failed.? Tensioning the blade more will always make the blade run further from the rim, but if the bearing has failed (quite common) then not much extra tension is required to make the blade track off.? Conversely reducing the tension will make the blade track toward the rim. Changing that bearing is a bear but quite do-able.
Then set it cutting something on the slowest speed and watch which wheel the blade comes off first.
If tracking is OK and the blade only comes off in the cut then most likely one of the blade guides is too far forward on its adjustment slot.? This acts as a pivot when the blade starts cutting and levers the blade off one of the rims (usually but not always the bottom rim).
Let us know how you get on. Rgds - jv
John Vreede