sounds like vaccum leak or blockage. the computer is receiving a signal that the engine is under load, even though it may not be, the manifold pressure lines (vacum lines) are sending a faulty pressure, this means the box wont change until there is "enough speed for it not to stall" or so it thinks.

Take off every little hose on the manifold and put it back on with some sealent or something that will prevent leaks in the future. throw away any with visible damage or cracks.

Did these things run throttle posistin sensors? it could be that aswell. but not sure