Vibration that comes and goes....

[jhandy]

I recently put new rims, tires and 7mm spacers on my car because the dealer said that my front rims where bent and unable to balance.
So....
From that time there is a vibration sometimes worse than before at about 70-90. Feels like the front tires are out of balance but how can the vibration come and go.
It is never gone all the way, but it get significatly worse from time to time.

Can both tires be out of balance and somehow cancel each other from time to time?

I cant bring the tires and rims back to discount tire to rebalance, because of there stupid policy.
They will not touch my car because it has spacers on it.
They fail to understand that they sold me the rims and tires, and told me it would fit, it did not and I was left buying spacers on my own.

Anyway, please advise.

James