Overheating causing tranny probs

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OK guyz I'm not great w/carz but I hav a book & I try, so I need some feedback on this. I hav a 2000 Chevy Malibu w/ a 3100. I hav recently had some overheating probs. The fan switch wuz replaced by a shop, yet coolant still kept pouring out the blow down hose on the resevoir. I wuz told it could B the resevoir not holding pressure so I replaced that. The prob persists. The shop told me the secondary fan is not coming on when it should due to an electrical problem so they rigged it 2 stay on constant. I'm not convinced, all though I'm no longer overheating or losing coolant & my temp gage reads nothing. Soon as they rigged my fan I started 2 feel my tranny shift very roughly. Tranny wuz pretty much (linkage needed minor adjusting) fine B4 the overheating. Now when I go from park 2 drive it shifts so rough I hear a big bang & feel az if I'm being rear ended. It does not do this in reverse only drive & it shifts thru every gear roughly jus not quite as bad as from park 2 drive. Can any1 giv me some insight? Any ideas on the overheating? Can the tranny B from the overheating prob or iz this jus an odd coincidence? I'm in the process of flushing out the dexcool & replacing w/green coolant & also in changing the thermostat. Tranny fluid & filter hav also been changed w/in the last year. Help please :(