A Shocking Question, Shock absorber lifespan Options

[Greg]

My '75 has Bilstein green shocks up front and Koni yellow's in the rear . The're aprox. 15 years old with around 100,000 miles no visible fluid leakage. The problem occures when I'm cruising at freeway speeds then get on the brakes then accelerate again, the car develops a vibration (after braking) that feels like the tires are bouncing up and down (undamped springs). The vibration goes away and the ride smooths back out whithin about 15 seconds. The brakes are all new, the tires are new and ballanced, wheels are true and the bushings are in good shape (including the Turbo Tie rods).
I think I know the answer to this already but I'm looking for confimation or other things to check.
I'm unsure of what an expected lifespan for these shocks should be because I have a 1985 BMW 535 that has the original Bilstein's and 180,000 miles and they stil work great.

If I do need new shocks, what's the hot set ups for a street car that occasionaly does autoX/ track?
The car is lowered, 140lb springs, 21mm bars up front, 205/50-15 all around, front sway bar.

Thanks in advance