Random No Spark, Starting problem!!

[Unlikely Hero]

Background on the car, its a 2g with a 1g swap with 1g head and an auto to standard swap. Basically i've been working on this car on and off for the last couple months when I've had the free time. Heres the problem

Car will run fine no problem at all. Then I'll turn her off for a couple minutes and when I go to turn her back on, she'll crank but won't turn over. Timing is good and the CAS is good but I don't understand why one minute its fine and the other minute it sucks. This has happened before and when I tried to crank her for awhile I'll get a really really loud pop. Last time this happened I basically didn't touch the car for a month and next time I tired to start it started right up fine. Ran good for a couple weeks and today same problem happened. Anyone have any suggestions where to start?

One question, does the transistor pack have to have a really good connection to the intake manifold for a ground?? Its on there but the mount can move. Thanks
=Jason-