AOZ Studio V1 (TBD bugs clean/-> V2)

In the following example, I'm trying to re-define Instr to allow for a negative startingPosition parameter.
The function works fine with any one or two of the following.
If, however, the function is called a third time, the program will freeze.

Function "instr",st$,fnd$,strt
    result=0
    If Abs(strt)<=Len(st$)
        If strt < 0
            startPos=Len(st$)-(abs(strt)-1)
            For p=startPos to 1 Step -1
                If Mid$(st$,p,Len(fnd$))=fnd$ Then result=p:Exit
            Next p
        Else
            For p=strt To Len(st$)
                If Mid$(st$,p,Len(fnd$))=fnd$ Then result=p:Exit
            Next p
        End If
    End If
End Function(result)

Any one or two of the following examples will work fine, but if you try a third one AOZ will freeze.

Print instr(inst$," ",1) ' result 5
Print instr(inst$," ",-1) ' result 35
Print instr(inst$," ",6) ' result 10
Print instr(inst$," ",-8) ' result 29
Print instr(inst$," ",40) ' result 0
Print instr(inst$," ",-40) ' result 0