So, was it just that the Romans lacked the proper institutions to defeat the Barbarians?
Short answer...People. In the west in the 300s there was a series of plagues that reduced the population. Tha and long border with the germanic tribes that the eastern empire didn't have.
"preventing the movement of labor, and excessive taxation, came from the Romans."
Roman taxation was based on land, if you didn't have the people farming on the land you didn't have the money to pay the taxes, hench laws were passed to prevent people from moving off the land. Remember the west started off with a lower population base than the east.
A couple of good places to start are
Thr Early Germans
Malcolm Todd
http://www.amazon.co.uk/Early-Germans-Peoples-Europe/dp/1405117141/sr=1-5/qid=1162615824/ref=sr_1_5/202-2666748-5772620?ie=UTF8&s=books
Barbarians And Romans
Justine Davis Randers- Pehrson
http://www.amazon.co.uk/Barbarians-Romans-Struggle-Europe-D-400-700/dp/080612511X/sr=1-10/qid=1162615932/ref=sr_1_10/202-2666748-5772620?ie=UTF8&s=books
Early Middle Ages
Philip Daileader
http://www.teach12.com/ttcx/coursedesclong2.aspx?cid=8267&id=8267&d=Early+Middle+Ages&pc=History%20-%20Ancient%20and%20Medieval
The Germanic invasions began late in the 2nd Century. As you recall, Marcus Aurelius died on the frontier, and was succeeded by his dissolute son, Commodus. The 3rd Century was bad for Rome, and it took the stern leadership of Diocletian is reestablish the authority of the state.