Try turning down the graphics including unit sizes to the minimum and see if the same thing happens. This will only remove the possibility that the load on your PCs is too much.
In some on-line game I 'lose control' over units - this seems to be caused by packet loss. So, data is being lost somewhere along the line (like a bad telephone line). In order to see if that is happening you need to work out the address of the target and run a path ping or similar.
The following shows that I was getting 100% packet loss in step 5 which was a BT owner server. It was only solved by switching internet providers (as BT India were not helpful in resolving the issue and even suggested during one on-line conversation on my PC that I should 'reset my Hub' which I'm fairly sure would have resulted in the on-line conversation being terminated).