Pro Tools 10 Error 9128 Running out of CPU power

When working with virtual instruments in Pro Tools 10, I'm starting to get error 9128 "You are running out of CPU power. Remove some Native plug-ins or increase the CPU usage limit in the Playback engine dialog". This happens when working with midi and virtual instruments. With just one virtual instrument assigned, this error occurs. I primarily use virtual instruments from UVI with UVI Workstation, Goliath from East-West, and VI.ONE from Vir2 Instruments. I increased the processors from 1 to 4 and then to 8 in the playback engine, but the error continues to appear. This error did not always occur, but has appeared more frequently in recent months.This is a dedicated machine for Pro Tools only, so not much of anything else running on it.



Specs:

Win7 Pro 64-bit SP1, Intel i7-3770 CPU @ 3.5 ghz, 4 core, 8 processor, 16 gb ram. Pro Tools 10.3.8 installed on a 1 tb 7200 rpm drive, all plug-ins on separate 1 tb drive 7200 rpm drive



Current Playback Engine Settings:

h/w buffer: 512

Host Processors: 8

CPU Usage Limit: 90%

Delay Compensation Engind: Long

Plug-in Streaming Buffer: Level 2

Optimize is not checked



Any ideas?