Heres my odd problems, and im pretty good with my volvo's this is #1 out of my 3.

I recently have had a string of problems with my 86 740 Turbo NO MODS, About 3 Months ago my head blew due to overboosting ( not known to me it was doing that ) anyways all is well had the gasket replaced didnt crack anything and the like, About 3 weeks ago i replaced the sensor under the #3 injector right behind the temp sender, all was well after that, i then replaced turbo hoses and some vacumm hoses, then the radiator went out replaced that and then this started, when i had the radiator out, i pressured cleaned the Intercooler inside and out looks good and no blockage there, broke the tranny cooler line replaced that,cleaned the engine throttle body grease around all areas ya know just cleaned her up, put the new radiator in and what when i tried engage the turbo nope all i kept hearing was pop pop pop it was backfireing into the turbo unit, so i figured hell with all the pips off for a week or 2 maybe the bearing got some rust on them, so i took the tubo off and rebuilt her like new, put everything back together no go same problem, also buring rich while at it, removed and replaced the black and crudded spark plugs only 2 months old with 800 miles on them thats odd, Checked all hoses again all is good in the hose area, everything appears to be good but still it isnt wanting to go when you hit the gas, at idle its perfect and when you rev the engine and do not engage the turbo all is well , once you engage the turbo while driving or hitting the gas hard in park it just goes nuts, i can hear the turbo spinning up and trying to boost the car but it just cant do it for some reason, hell i can hear it spinning to what id consider full spin. Any ideas here would be greatly appricated really it would.