rough idle in an 80 280ce

[moxwell]

I just bought a 1980 280ce which has a very rough idle. I took the car in to get a tune up thinking it was a bad spark plug (not my usual import mechanic, however he is a trusted mechanic who works on my 65 mustang), and he said i probably had an intake leak somewhere and that it may be a bad gasket. He sprayed a little carb spray on the rear of the block and the car settled down nicely afterwhich a few minutes it went to a rough idle again. The car does not smoke at all and has a relatively low 130,000 miles on it. He changed the oil and reported it was all oil. I was just wondering if anyone can offer any insight or suggestions.