Engine over-heating; runs rough; back fires

1991 Buick Skylark 2.5 sedan 89,000 miles. When I bought it, it had a cracked manifold. I had that repaired. It ran very smoothly and obtained 28 miles per gallon fuel economy. Then last winter the radiator sprang a leak. I had that repaired. When we picked the car up from the radiator repairman, he commented that he thought we should get a "tune up" on it because it was running so rough he could barely test drive it after the repair. Indeed, it ran and idled incredibly rough and now the heater was not giving heat. I talked to an auto mechanic who said he thought that the radiator repairman, when he flushed the radiator, had plugged some of the conduits (tubes or whatever) that allow the engine heat to transfer to the heater. (I may not be reporting this accurately, as I did not quite understand what he was describing to me). I talked to the radiator repairman who denied any responsibility and asked if I was accusing him of careless work. The weather by now has warmed up. I took it to an auto repair shop for the "tune up" suggested by the radiator repairman. There the mechanic put in new spark plugs, distributor cap and rotors, replaced the air filter and a pinched air hose. It still ran rough and was now overheating. He suggested that we replace the thermostat, which we did. It runs rougher than ever. I cannot drive more than about thirty miles without the coolant boiling or sometimes blowing the reservoir. It runs so rough now that it is losing power going up the steep hills in our town, barely making it to the top even with the accelerator fully depressed. When I press down on the accelerator to gain more speed or power, I get less. At one intersection, whenI pressed the accelerator to merge into traffic I was nearly broad sided because of sudden loss of power. Besides the loss of power there was lots of backfiring in the exhaust system just like there is when going up hill. Putter putter putter POW POW putter putter putter POW POW and then (sometimes) a little more power. I am at my wits end to know what the problem is or to whom to take it for repair. I am only getting about 12 miles per gallon now and can still only drive about thirty miles before stopping to let it cool completely. What now? What do I do?