Did it in Brainfuck! I had a bit of trouble generating random numbers. I hoped something simple like a LFSR could give me a fairly decent looking sequence, but I found it looped too quickly and gave boring output. I resorted to using a RNG found here, which actually takes quite a long time to run. Since BF has no capacity to get any source of entropy from the operating system, I first have the user input any text (I pound on they keyboard) to seed the RNG. After that it takes the number of lines and outputs random BF, using 16 hard-coded subsequences.
get random seed input till newline (10)
>>>+[,[<+>>+>>>>>>+++<<<<<<<-]>[<+>-]<----------]
get number of lines
<<<,[->>
Map: n nesting seed lineCount _ _ _ _ _ _ randh randl
mod by 32 to get char count for this line
[>+>+<<-]>>[<<+>>-]>++++[<++++++++>-]<<
[->-[>+>>]>[+[-<+>]>+>>]<<<<<] >[-]>>[-]<[<<+>>-]
loop that many times
<<[-<
generate next 'random' number
>>[-]>[-]>[-]>[-]>[-]>[-]>[<<<<<<+>>>>>>-]>[<<<<<<+>>>>>>-]<<<<+++++++
[<+++++++++++>-]<[<<[>>>>>>+<<<+<<<-]>>>[<<<+>>>-]<<[>>>>>+<<<+>+<<<-]
>>>[<<<+>>>-]<[>>>>+[<<<+>+>>-]<<[>>+<<-]+<[>-<[-]]>[>+<-]<<-]<-]+++++
+[>++++++++<-]>-[<<[>>>>>+<<<<+<-]>[<+>-]>-]<<<[-]>[-]+++++[<+++++>-]<
[>>>>>>>+[<<<<<<+>+>>>>>-]<<<<<[>>>>>+<<<<<-]+<[>-<[-]]>[>>>>+<<<<-]<<
-]++++++[>>>>>>+++++++++<<<<<<-]>>>>>>[<<<<<<<<+>>+>>>>>>-]<<<<<<[>>>>
>>+<<<<<<-]<<
seed is now next number in sequence
copy and mod 16
[>>+>+<<<-]>>>[<<<+>>>-]>++++[<++++>-]<<
[->-[>+>>]>[+[-<+>]>+>>]<<<<<]>[-]>>[-]<
now we have bottom 4 bits of data; pick an output sequence:
#00: +
[<+<+>>-]<[>+<-]+<[>-<[-]]>[-[-]>[-]<
>++++++[<+++++++>-]<+.[-]]>
#01: +
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-[-]>[-]<
>++++++[<+++++++>-]<+.[-]]>
#02: -
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-
[-]>[-]<
>+++++[<+++++++++>-]<.[-]]>
#03: [->-[>+>>]>[+[-<+>]>+>>]<<<<<]
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-[-]>[-]<
>+++++++[<+++++++++++++>-]<.
>+++++[<--------->-]<-.
+++++++++++++++++.
-----------------.
>+++++[<+++++++++>-]<+.
>++++[<------->-]<-.
>+++[<------>-]<-.
>+++[<++++++>-]<+..
>+++++[<++++++>-]<+.
>+++++[<------>-]<-.
>++++[<+++++++>-]<+.
>++++++[<-------->-]<.
>++++++[<++++++++>-]<.
>+++++[<--------->-]<-.
+++++++++++++++.
-----------------.
>+++[<++++++>-]<+.
>+++++[<++++++>-]<+.
>+++++[<------>-]<-.
>+++[<------>-]<-.
>+++[<++++++>-]<+..
>+++++[<++++++>-]<+.
>++++[<-------->-]<-.....>++++[<++++++++>-]<+.[-]]>
#04: <
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-
[-]>[-]<
>++++++[<++++++++++>-]<.[-]]>
#05: <
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-
[-]>[-]<
>++++++[<++++++++++>-]<.[-]]>
#06: >
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[- ----[>+<----]>---.[-]<]>
#07: x y z
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-[-]>[-]<
>++++[<++++++++>-]<.
>++++++++[<+++++++++++>-]<.
>++++++++[<----------->-]<.
>++++++++[<+++++++++++>-]<+.
>++++++++[<----------->-]<-.
>+++++++++[<++++++++++>-]<.[-]]>
#08: [
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-
[-]>[-]<
>+++++++[<+++++++++++++>-]<.[-]<<<<+>>>>
]>
#09: [
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-
[-]>[-]<
>+++++++[<+++++++++++++>-]<.[-]<<<<+>>>>
]>
#10: ++[>+<------]>
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-
[-]>[-]<
>++++++[<+++++++>-]<+..
>++++++[<++++++++>-]<.
>++++[<------->-]<-.
>+++[<------>-]<-.+++++++++++++++++.
---------------......>++++++[<++++++++>-]<.
>+++++[<------>-]<-.[-]]>
#11: +<->++
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[- [-]>[-]<
>++++++[<+++++++>-]<+.
+++++++++++++++++.
---------------.
+++++++++++++++++.
>+++[<------>-]<-..[-]]>
#12: .
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-[-]>[-]<
>+++++[<+++++++++>-]<+.[-]]>
#13: ,
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-[-]>[-]<
>++++[<+++++++++++>-]<.
[-]]>
#14: [<+<+>>-]
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-
[-]>[-]<
>+++++++[<+++++++++++++>-]<.
>+++++[<------>-]<-.
-----------------.
+++++++++++++++++.
-----------------.
>+++[<++++++>-]<+..-----------------.
>++++++[<++++++++>-]<.[-]]>
#15: [-]
-[<+<+>>-]<[>+<-]+<[>-<[-]]>[-[-]>[-]<
>+++++++[<+++++++++++++>-]<.
>+++++[<--------->-]<-.
>++++++[<++++++++>-]<.[-]]>
[-]<<<
]
>+++++++[<+++++++++++++>-]<++<<[>>.<<-]>>[-]<<++++++++++.[-]<
]
Output for input
qqq
10
is
[<+<+>>-]<[-] x y z.< x y z<.++[>+<------]>[[-]+<. x y z++[>+<------]>.+<->++[->-[>+>>]>[+[-<+>]>+>>]<<<<<][+[<+<+>>-] x y z[<+<+>>-][<+<+>>-][<+<+>>-]++[>+<------]>+<->++]]
-[->-[>+>>]>[+[-<+>]>+>>]<<<<<][<+<+>>-][-][ x y z++[>+<------]>,[-][->-[>+>>]>[+[-<+>]>+>>]<<<<<]-]
<[->-[>+>>]>[+[-<+>]>+>>]<<<<<]
+,++[>+<------]>-[[-]+<->++[[->-[>+>>]>[+[-<+>]>+>>]<<<<<],++[>+<------]>[->-[>+>>]>[+[-<+>]>+>>]<<<<<]++[>+<------]>.[+ x y z+<]]]
++[>+<------]>++-[-]-[-]+,[->-[>+>>]>[+[-<+>]>+>>]<<<<<][->-[>+>>]>[+[-<+>]>+>>]<<<<<][->-[>+>>]>[+[-<+>]>+>>]<<<<<]+,+[<++[>+<------]>,-+<->++]
-[<+<+>>-]-., x y z[ x y z<[<+<+>>-]< x y z<[+<->+++<->++-++[>+<------]>+<->+++[[<-[-][+<->++]]]]]
[<+<+>>-]++[<+<+>>-]-<<[-]+<->++<[]
,[->-[>+>>]>[+[-<+>]>+>>]<<<<<]++[>+<------]>++<->+++<->++[<+<+>>-]<
++[>+<------]>[<+<+>>-][+<->+++<->++[[->-[>+>>]>[+[-<+>]>+>>]<<<<<]<[<+<+>>-]++[>+<------]> x y z x y z<..[[-][<+<+>>-],[<+<+>>-]+,]]]
-, x y z<<[<[->-[>+>>]>[+[-<+>]>+>>]<<<<<].[,<-]]
and it runs in about 2 minutes on my box. Output contains entirety of BF syntax, including comments, and it well formed.