Vauxhall Astra loss of power

can be driving along then it starts to loss power then cuts out service light comes on , but then after a few mins it starts with out any problems, had it looked at fuel filter in tank was cleared from black sludge then went ok for a few days then did the same loss of power, the garage have replace a few bits such as filters near the fuel pump crank shaft sensor, and pipe, seams to be ok but then on the third try went out and lost power again, now hes trying to clean out or possible replace the fuel tank to see if this helps , its ok when doing short runs but i do about hour to work and an hour back so on long runs it seams to struggle with the power it just stops no warning lights come on othen than when it stops .......... what the garage did notice was the fuel tank looks if its been repaired at some point any ideas ,iam running out of money getting it fixed .