Just a follow up to my previous post. The attached link is to an MGA site and explains very well how the traditional coil/condenser/points systems works and the importance ot the condenser in the system. Its for a 12 volt system but the principles apply to our 6 volt system. The author indicated elsewhere that a bad condenser may allow the car to run at idle but not at higher rpms. This is exactly what happened to me and your symptons sound the same as mine were. Condensers are cheap so try a different one before assuming that more expensive problems to fix may be the problem.
A quote from his site goes something like this:
"90% of all carburetion problems are electrical"
In my case it was oh so true.
http://mgaguru.com/mgtech/ignition/ig108.htm