David:
Sounds like your carbs are out of sync. Mine ran REALLY rough when I first bought it. The car would run OK as long as I kept the RPM's around 3000. However, if I jumped on the accelerator real hard, it would cough and sputter.
I took all 4 carbs off, rebuilt them (they needed it too!), and re-installed with new base gaskets. It really improved things! At the same time, I also replaced the spark plugs. The Jalpa can foul plugs real easy if the carbs run rich.
I've been using the Autolite 4063 in my car. I've also used the NGK B8EV.
Another item the really helps (although I've not installed it yet), is replacing the stock Bosch ignition box with the PermaTune and coil. Everyone that has installed it, has been very pleased with the results. I'll be installing one in mine shortly. Here is a link to their page:
http://www.perma-tune.com/931902.html
Good luck!