need help on parallel.for loop

[tang3li2]

hi i'm currently facing race condition in variable, i don't know how to solve this event..
particulary in race condition,

i have code that cut and replace if the letters has been found
example i have a word "cat", if i found the letter "a" it will be removed to the word "cat"
so the remaining would be "ct", repeat the process until it will completely done..

so i have created it and already fully functioning in 1 thread..
but when i transfer it on a parallel.for loop, it acts weird, i dig in deeper and i found out that the process is to fast
and it overwrites the variable, so i try the synclock method, but it slows the process.. and i try the interlock but no good it acts weird because it doesnt process the ordering like "1.2.3.4.5", what i want to do is to preserve the speed without using synclock..

so here's the code.

sqrt = Math.Ceiling(Math.Sqrt(13599))
        a = 0
        xchar2 = ""
       Parallel.For(1, sqrt + 1, Sub(o)                                
                                 Dim a, b, c, i, y, z As Long
                                 i = 2
                                 a = 0
                                 If o = 1 Then
                                     b = 1
                                     c = sqrt
                                 Else
                                     b = ((o * sqrt ) + 1) - sqrt
                                     c = (o * (sqrt ))
                                 End If
                                 For y = b To c
                                     a = y
                                     tmpChar = "cat"
                                     For z = 0 To i
                                         xmod(z) = a Mod 26
                                         a = ((a - a Mod 26) / 26)
                                         If InStr(tmpChar, numToletter(xmod(z))) = 0 Then
                                             bListed1 = False
                                             Exit For
                                         Else
                                             tmpChar = Mid(tmpChar, 1, InStr(tmpChar, numToletter(xmod(z))) - 1) & Mid(tmpChar, InStr(tmpChar, numToletter(xmod(z))) + 1)
                                             If z = 0 Then
                                                 If check2(numToletter(xmod(z))) Then
                                                     xchar2 = numToletter(xmod(z))
                                                     bListed1 = True
                                                 Else
                                                     xchar2 = ""
                                                     bListed1 = False
                                                     Exit For
                                                 End If
                                             Else
                                                 If check2(numToletter(xmod(z))) Then
                                                     xchar2 &= numToletter(xmod(z))
                                                     bListed1 = True
                                                 Else
                                                     xchar2 = ""
                                                     bListed1 = False
                                                     Exit For
                                                 End If
                                             End If
                                         End If

                                         '---
                                     Next
                                     If bListed1 Then
                                         Debug.Print(StrReverse(xchar2))
                                     End If
                                 Next
                             End Sub)

and this is the part where the race condition occur, it produces error that it should be equal to zero or greater than zero, because the variable is less than zero or null ( i guess )

tmpChar = Mid(tmpChar, 1, InStr(tmpChar, numToletter(xmod(z))) - 1) & Mid(tmpChar, InStr(tmpChar, numToletter(xmod(z))) + 1)

i wonder how to implement the Interlocked method here??? i think of it as a preliminary solution, any suggestion/comments guys, any related topic on parallel.for loop is a great big help.. thank you in advance.