Had this before with K-Jet 3i, I took it to a place to get the fuel and fuel control pressures checked. Came back okay and it fixed itself, never happened again. It is quite frustrating because I changed injectors as well before going to the garage.

Try and find a good old school mechanic to check control pressures but before that check dizzy cap very carefully for cracks and change leads as well.

I have known new spark plugs to be faulty so perhaps a new set of plugs (if it does not fix it they will come in for the next service so no loss).

edit, just thought could be old crappy fuel or water in the fuel system.