There will be a check valve that tries to hold built up vaccum in the booster. Sounds like it might be wide open. Will be located outside the booster. Seems like at full accumulated vaccum pressure brakes work well. High engine speed is making good vaccum. At lower speeds vaccum is low. Pump cannot develop enough. Or it is flowing back out of booster to equalise. Might also be from leakage of vaccum somewhere. To test the booster for small internal leak get hold of vaccum gauge with pump up feature. The booster should be able to hold constant vaccum. But first find out if the check valve in that line is closing. Nice if that is all it is. After problem is repaired check vaccum pressure at idle for factory specs. Chase down any additional leakages if any. If you own one of the 123s a vaccum testor is a good tool to own. The rebuild kit for the pump is cheap on the earlier models like you have as well but make sure you need it first. Check valve in vaccuim pump can be marginal as well but again with tester you can isolate whatever is causing the problem. A real good ideal is to check the previous information in the archives out as well.