2001 Infiniti QX Car engine shakes whiile accelerating

Shakes or Wobbles problem
2001 Infiniti QX 6 cyl Automatic 130000 miles

I had the timing belts on my car replaced a mth ago at a lube shop. I had no problems till recently when my engine started to shake sometimes. It doenst shake regularly, but once on the highway, every time I went past 90 kms it would shake and then I would have to ease off on the throtle, and every time I accelerated it shook. On my way back the next day, it worked fine at the highway at speeds of 120 k's. Recently it started to shake more often, even at lower speeds. It doesnt happen all the time, but randomly. Any help would be appreciated. Thanks.