jump which way? sideways? if the rocker is not loose as you say, then is the valve loose? that puts it back to considering the valve stems slopping around in loose valve guides, new or not. 

Or, if jumping you mean hanging up and then moving more quickly closed,  that still implicates a sticking valve. Either that or the closing ramp on those two cam lobes is worn. 

Look closely, the valve springs aren't broken are they?  

Better not be!  I had the head rebuilt this spring with new valves, springs, and guides.

EDIT:  Just checked.  The rocker does not get loose before it snaps closed.

Valve sticking in the guide.

My engine developed a loud tick day before yesterday. 

I ruled out the fan and alternator.  I don't think it's rod knock or piston slap as it doesn't change when pulling spark plugs off.  

I've just been adjusting the valves and noticed something odd.  The number 6 and 8 valves appear to jump when closing.  This jump is accompanied by the tap.  Is this an indication that the tappets are getting stuck?  Knackered cam maybe?

The engine is slated for rebuild/replacement when the snow starts flying, due to low oil pressure and inconsistent compression.  I'd like to keep it running until then.

Thanks in advance.