i would suspect a cracked plug, when installing them sometimes the socket binds on the plug and makes a hairline crack and causes the same conditions.....i have had that before....

a week spring would not cause the issue, it would just bleed off the pressure IE..running fat...

but keep us updated on your results since you've taken it apart...

a bent reluctor ring would pretty much pop the same but on higher rpms.... i had one of those problems too, but it would do it at 5500-5800 all the time...

x2 on quik, i would do a .030 or so gap...unless running high boost 20 + i would do .025-.028