125-200cc Head shake and speed wobbles

I took my '06 Cr125 to the MX track Saturday and almost got spit off from a violent speed wobble. Also got head shake on the fast straight. Now, I'm old (according to my kids) and know next to nothing about modern-ish suspension. Where do I start looking to fix this?
This bike has 45mm Marzocchi dual-chamber forks with some valves from a Canadian guy and .38 springs, and an Ohlins shock with the stock soft spring.

Thanks

need to do all the steps or at least some. tire pressure, well balanced tires/wheels, steering bearings, suspension settings=sag and clickers, chain

Is the bike sprung correctly for your weight? If the forks are too soft, they will ride lower in their travel and you will get the death wobbles....
If sprung correctly, try going in on the compression. 2-3 clicks at a time. Going out on the fork rebound may also help...

ditto on the head bearings and tire pressure. check and retorque axle, pinch bolts too.

Good god I shudder at the thought of .38 springs for ANYTHING, let alone for moto. I would have to guess that the front end is riding low in the travel, especially if you are heavier. This will aggravate headshake.

Anyway, What RT said, with special attention to the head bearings. The top nut on my triple clamp will slowly back off on my bike, did on my older-framed Huskies also. I run a sharpie mark on the nut and tripleclamp so I can tell at a glance now. Before the mark, I used to always know because I'd get a scary warning and my normally rock-steady front end would give me a tankslapper and sure enough, the nut had loosened. Run the bearing nut (under the top clamp) on the tight side, you'll have to experiment to find the right tension as locking down the top nut increases bearing pressure. You want the front end to still self-center while riding with no input, but it should feel sluggish with the front end in the air on a stand. Also don't overlook tire pressure.

You might also check how far your forks are up through the triples...I pushed mine up through to much and got a scary high speed tank slapper.

Yeah definitely check out the steering stem area for potential loosening. If the nut on top of the steering stem that's right under the top clamp gets loose, at all, you will have some issues. Especially under heavy front breakage... I use a dab of fingernail polish to spot bolt creep in that area especially.

1mm will make a HUGE difference.