Backtesting reliability isuues

Dear all,
Lately I've encountered some backtesting problems.
Testing my system with data downloaded from http://www.alpari-idc.com/en/dc/databank.php form the beginning of 2007 till now gave a 1,500 pips profit.
Reinstalling of Metatrader, new import and period conversion of the history data gave a totally different outcome - only 100 pips.
This is not the first time of inconsistency in backtesting I\m encountering with.
Has anyone experienced similar phenomena?
Can backtesting be accountable?

I have the same problem, in forward test I made 385 pips for last two months, with strategy tester show me only 5. If I use historical data from alpari the results is more wrong.
I think that there are somebody in this forum that make very good backtesting. If he read this thread maybe can help us. I think his nick is Timmer.