Had a similar problem after fitting new plugs leads and rotor arm on our Sierra, turned out to be the new NGK plugs which I had fitted, there was a fault with them and gave a very naff spark, fitted the old Motorcraft plugs back in and got a nice fat blue spark again.
In a separate incident a friend of mine had re built a Triumph 5SW and he could only get 10 miles or so before it would start missing or if stoped would not start again, he also had a NGK plug in it, after I told him of my problems he changed it to a Champion and has not had a problem since.
Check the plugs.