Try to disable your secondaries on your carb. If it goes away, your seconds are causing the engine to stall slightly then start to rev up. This is a common thing on alot of four barrels with an improper tune.
If you stomp the gas, does your car hesitate and then a split second later come on like gangbusters? If so your seconds could be opening up too soon.
Just wanted to point that out since alot of times a problem in timing can be caused by fuel and vice versa. They should always be thought of as one system, in my opinion.
Also, I dont see anyway reason why the timing would retard and then come back up unless the engine was actually slowing down. Matter of fact, now that I think about it, if you have the vac. advanced disconnected then the weights should be riding on the pins. I could be wrong about that though. If thats the case, then I would think the springs would be too light.
What kind of distributor is it? Is it a factory mopar? If so, and your trying to get the timing to come in sooner with lighter springs, you may want to look at how much timing is "in the distributor" compared to how much your actually wanting. You could possibly take some of the timing out of the dist. with some bushings on the pins. However, I personally have never done that or read/seen it done on a mopar dist. Best bet would be to get a timing tape, look up DIY mopar dist. recurving, and either weld up the slots for the pins or use the cheap sams method of heating up the end of each slot, banging it with a hammer and 3/8th bolt, then grinding the slot back with a dremel until you get the timing you want. I have done that before, and it works really well.
EDIT: IDK if the OP checked all these things already, I just put that up cause its alot easier to check those things (along with the shaft riding up) before tearing into the timing chain.
