Extracting Sequential Pattern

Can someone advise/help me on how to write a script to extract sequential lines. I was able to find and get a script working to create permutations of the inputs, but that not what I want/need.

awk 'function perm(p,s,     i) {
       for(i=1;i<=n;i++)
         if(p==1)
           printf "%s%s\n",s,A[i]
         else
           perm(p-1,s A[i]", ")
     }
     {
       A[++n]=$1
     }
     END{
       perm(n)
     }' infile

Unfortunately, i dont understand the script well enough to made modification (not due to lack of trying). I need to extract 2 to 5 sequential lines/word patterns.

An illustration of what i need is as follows:

Eg.

inputfile.txt:

A
B
C
D
E
F
G

outputfile.txt:

A B
B C
C D
D E
E F
F G
A B C
B C D
C D E
D E F
E F G
A B C D
B C D E
C D E F
D E F G
A B C D E
B C D E F
C D E F G