Mustangs stalls when clutch pushed in @Higher RPM

I've had an issue with my 2004 Mustang GT 5-speed for a while now. Heres a scenario. I'm driving down the road accelerating in third gear, the RPM's get to 3000rpm and I decide to shift to fourth gear. If I hold down the clutch without actually engaging into 4th gear, the rpms drop (like they should) but the needle will fall to zero and the engine dies. Normally I would engage the clutch and down shift or whatever, but the engine should only drop to its set idle right? This problem normally only happens at speeds above 40mph. The car idles fine, and starts fine (most of the time). The car only has 23,000 mile on it and and everything else seems to work fine. But sometimes the engine stall catches me off guard, and since the power steering and pwr brakes shut off, it could be dangerous if I can't pop th eclutch fast enough to get the engine going again.
Do you have any suggestions or anyone had this problem before?:headscratch: