Bike: 2008 Specialized Ruby Comp Compact

As I’m cruising along on the bike, out of the blue it will feel like the brakes have engaged and I get lurched forward- sometimes pretty hard. I can continue to pedal, but as I’m pedaling- it feels like I’m riding with the brakes slightly on or that I’m riding through sand or mud (sluggish). The brakes are not touching the rims. There is no warning and no noises from the chain or anywhere else. I’m in the large chain ring and this has been occurring in both low and high gears. At first this was random, but it is occurring more frequently now. I have been able to recreate the situation at low speed and low gear. I’ve had the bike for just over a year w/ 700 mi (stock components) -and have never had any problems up until now.

My bike shop has been trying to help resolve the issue, but they have not been able to recreate the problem (which adds to my frustration). So far we have:
1. made adjustments with the brakes to make sure they weren’t rubbing the rim (wheels are still pretty true) 2. changed wheel sets 3. upgraded the bottom bracket

I think we are running out of things to replace. The chain tracks good (no rubbing).
Has anyone heard of or dealt with an issue like this? Any suggestions or advice would be greatly appreciated.
Thanks in advance.