I have a feeling that it is your optispark... I know you say it has been replaced, but shops often overlook small details...
specifically, if you switched to the vented style when you replaced the opti, it will now be driven by the cam dowel pin... the 94 setup has the timing gear with center splines and a "coupler" which connects to the opti... Since the new opti is driven with a dowel pin, the shop who did the vented opti conversion had to install a longer dowel pin...
A VERY VERY common problem people have when they install the vented optis is that they fail to check the exact dowel pin length... they assume that as long as it sticks out in the opti and spins it, then the install is done... WRONG... If the dowel pin is even a few throusanths too long, it will bottom out in the opti and cause damage too it... sometimes it takes a month for this to show up, sometimes it shows up within a couple of hours, all depends on how "too long" it was...
The GM spec for the dowel pin length is .620"... some have used .650" with no trouble, and others yet have claimed to use .685"... I would assume .685" is probably the max, but it needs to be checked on all optis... the goals it to make sure the dowel pin does not bottom out in the opti... I would set it to .620" to be safe... that will get it plenty far into the opti to drive it, and have no risk of bottoming out...
if you have a garage do the work, have them measure the length of the dowel pin from the face of the cam to the tip of the pin... if it is too long, that is your problem, and they can fix it by using a grinder to grind the dowel pin down to the correct length...
-Garrett