I've been noticing that my GSR loses low-end pep once it warms up. While doing a coolant fill and waiting for the engine to reach operating temperature, I pulled the vacuum hose off the intake manifold's secondary runner dashpot. There was vacuum, and the dashpot reacted.

However, once the engine did reach operating temperature, I pulled the vacuum hose off again, and the dashpot didn't response (was remaining in the "open" position) and there was no vacuum in the hose.

Ideas?