If it's a diesel and still has good compression.I would think it's either a timing problem with the IP,or a fuel flow problem.If you'r asking about a gasser,I would start checking all the vacuum hoses.Don't just glance at them.Pull each one off individually and replace any that aren't perfect.My wifes gasser Cabriolet convertible had the same problem this spring when we took it out of winter storage.I had to replace almost half the hoses.Lots of small vacuum leaks.