Taken from the odl Pinrepair guides for WPC games but the theory will be pretty much the same I am pretty sure.
If the flipper works, but...
Non-Fliptronics Games:
Any one of the flippers flutters (goes up, comes down, goes up, comes down),
when the cabinet flipper button is held in (the flipper flutters slowly). On a
non-fliptronics games, this is a problem with the EOS switch. The EOS switch
for the offending flipper, which should be closed when the flipper is deenergized,
is not making good contact. Either the switch contacts are
misadjusted or burned, or an EOS switch wire is broken. Or it could mean the
hold winding on the coil itself is broken. The hold winding on the coil is the thin
wire. If it is broken, you can usually see the wire has broken away from one of
the solder lug. Test the coil (see above) with a DMM. Sometimes the break
can provide an intermittent connection too.
Flipper flutter could also be the EOS switch is not adjusted properly. If the
moving EOS switch blade does not have enough tension against the other
switch blade, flipper flutter can occur. Sometimes adjusting the EOS switch
with the game on and the flipper button held in is the best way (but be careful
not to short the high voltage EOS to another switch!), because wear in the
flipper linkages can give wrong EOS switch measurements when moving a
flipper bat by hand. Also check the cabinet switches for proper tension and
that they are clean. Lastly, try replacing the coil stop. A very worn coil stop
can cause flipper flutter
