1990 Jaguar XJS sluggish, low low idle, won't go.

Hi, I was driving my xjs yesterday at about 50 mph when suddenly there was a loss of power. Now, it idles very low and is very very sluggish - will barely go above 20-30 mph. Last year it seemed to do the same thing after a wash but started and ran fine the next morning. The car had a service over a year ago - new injectors, etc.

Thanks much!