I plug all my flat tyres and never had a problem, even at top speed.
I use the same repair kit as the tyre-dealers (string impregnated with some sort of vulcanising glue), and a mini pump. At almost no cost and 10 min later and I'm back on the road.
I even drove a completely flat back-tyre for 5 km, no problem.
Redbird, what exactly happened when the plug let go?