Tach jump with engine stumble. I'm Clueless...

[LazerSquid]

Hey everybody, I'm new to this forum and hoping you guys can help me out with a weird problem that popped up. for the past week or two, I've had a slight jump in my tach, but otherwise the car would run fine. Now though, the engine gets caught on the way down around 1500 rpms, and the tach just flops around wildly while the engine stutters like somebody is cutting and reconnecting power or something. I recently replaced the cap and rotor, and they're working fine. I logged a 30 second portion from my wide band controller and the AFR spikes every time the engine stumbles or makes a big jump, which rules our pre-detonating or mis-fires, or it just being a silly tach problem. I'm starting to wonder if it could be my coil shorting out? It only seems to happen when the car has been running for >5 minutes, so I'm thinking it's related to the heat. Any help would be great! I'm just praying it's not my DME...

Here's some things I've done recently:
Lucas Injector Cleaner
Sea-Foam in crankcase for 10-miles then oil change with 20W-50
New cap&rotor
Cleaned contacts on tach
Cleaned both ignition coil contacts
New alternator (voltage regulator went out on old one)
All new vacuum lines
new fuel filter and hoses
New AFM
New ICV
Innovate Motorsports LC-2 Wideband O2
And I'm probably forgetting some other things. I've always run 91oct gas, and the problem has persisted through multiple tanks, so I know it's not just bad gas. Any help would be great!! Thank you for your help in advance!

EDIT: Forgot to mention; I tested the resistance on my coil, and it cam back with 1.3ohm primary and 7.6k secondary.

EDIT II: Also forgot to mention, this is a 1984 318i with the L-Jetronic ECU

[Kevinl]

Cps

[LazerSquid]

the 1984 318i doesn't have a crank position sensor :/

[Gregs///M]

If an inaccurate or jumpy tach is your only problem, it is most likely dead SI board batteries. See my thread or website for tons of info about the BMW SI board.