int pcount1 = Convert.ToInt32(pcount);
You are probably throwing a FormatException on the above line now if the string contains more than 0-9.
FormatException - value does not consist of an optional sign followed by a sequence of digits (0 through 9).
If you know the first 4 chars will always be "XXX-" (e.g. "MEP-") then why don't you:
int pcount1 = Convert.ToInt32(pcount.Substring(4)); // ######## part
int pcountAdd = pcount1 + 1; // ######## + 1
textBox1.Text = String.Format("{0}-{1:D8}", pcount.Substring(0, 4), pcountAdd);
Which will convert only the numeric part, increment it, and then concatenate it with the first part.
Also you can look into the TryParse method. Which will indicate if your parse succeeded.
TryParse - Converts the string representation of a number to its 32-bit signed integer equivalent. A return value indicates whether the operation succeeded.