help with 71 beetle

Can anyone offer advice on how to get our beetle to run smooth
we got all the electrics updated but under load it misses
when cold its not to bad
but after 10 minutes its a real pain
Ive had it at the mechanics and it runs ok for a week or so
just don't know what else to do
should I change the coil?
that's all I have left
or maybe renew the dizzy?

Are you running points or electronic ignition? I have had a few electronic ignition modules go over the years, and that is how some of them go.