join all lines inside pattern that occurs more than once

[Nikolaos A. Patsopoulos]

Hi,

I want to join all lines that are inside a given pattern and occurs more 
than once  in the text, ie:

....
PatternStart ....
....text1
....text2
...text3
..text4
PatternEnd
...
...


PatternStart ....
....text1
....text2
...text3
..text4
...
...textn
PatternEnd
....


...
PatternStart ........text1....text2...text3..text4PatternEnd
....
....

PatternStart ........text1....text2...text3..text4...    ...textnPatternEnd
....


I tried to use:
:g/PatternStart\_.\{-}PatternEnd/ J

but this joins only first and second line of the pattern.

How can I tell vi to join all lines inside all occurrences of this 
pattern with variable containing lines?

Thanks in advance,

Nikos