99 tacoma coolant reservoir spews

I have a 99 3.4, a couple of times now on roadtrips, out of nowhere the reservoir just spews while I'm driving, but the truck temp stays the same and the radiator is still full. The first couple of times I thought the Res. Cap had come off because u can see the coolant spewing out onto my Hood between the Hood and flaired fender. The cap is definitely on, so why do I keep spewing out of the reservoir?????? It only happens on the highway and usually at high rpms above 70 MPH.......... help please.

 

Your truck is overheating, and the cap is at max pressure, releasing the coolant in to the resevoir.

Check your heads in the back, by the firewall. That's where they usually start to leak. Also check out your water pump for leakage.