var stacks:array[1..1000] of string;
    n,i,j,t1,t2,l:longint;
    err:integer;
    st:string;

That's all. Why MLE10? In every 4 bytes of a string i write integer and on 'pop' recover it. So the sum of the lengths of the strings can't be more than 4*100000 bytes. Plus some bytes for compiler is approximately 700 kb. Why it takes more than 800?????

string is a dynamic array, Pascal can allocate memory for it with some reserve. I think its your problem