After driving for a while I always fall through the ground into the sea and then after a few seconds I'm back on the ground again but with no vehicle!

It's really annoying and is making me fail alot of the missions. I'm on the one when you get that assault riffle and have to chase a car a shoot other cars, but with this bug it's impossible.

Anyone else have this?

It started to happen after I installed the High Rez texture patch, but I'm not sure if this is the cause. The rest of the game runs very smoothly.