After 2 rebalancing jobs, it sounds like you have a bad tire. Might check the rims to be sure that they are straight just to be sure but my money would be on one or both front tires.
The vibration does take the fun out of driving doesn't it. You are much less likely to have a problem with better tires.
EDIT
You don't say how many kilometers you have on the car. I started with the vibration on my stock Grodyears at 14,000 miles. Started at about 110 Km/hr then got worse to the point that vibration would set in about 90 Km/hr. By that time the tread was almost gone anyway, so I replaced all four tires at 16,000 miles with Yokohoma's and haven't looked back.
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2003 Almond GT AutoStick, body colored rear splash guards and hood struts, chrome gear shift, AC & vent knobs, billet steering wheel spokes and pedals, AMX1397 Turbo-Intake Pipe.
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