1999 Mercury Sable V6 51,000 miles

This car has been well maintained, I take it in twice a year back to the dealer for maintenance. Only drive it mostly 3 or 4 times a month as I live in the city and take it out for road trips, maybe 5000-6000 miles a year. Last time I went to the dealer I complained that the car felt like it took a long time to shift between 40 - 45 mph on an incline only. Dealer mechanic said transmission dying. Been driving it a coupla months now and don't experience same sluggishness. Took it to transmission specialist who says nothing wrong with transmission. Dealer mechanic also said coolant system had leak and put in dye. Transmission guy says no leak. Dealer mechanic said catalytic convertor going, too. Transmission guy says car rides quiet, would make much more noise if that was going. Dealer said repairs would be $1500 transmission, $2500 catalytic convertor, $400 coolant system, suggested buying new car. Question 1: can the transmission experience the problem I noticed after a long winter of few drives, and once car was used more frequently, perform better? Question 2: sometimes when I hit a bump I hear a squeak in the front right of car? What could that be? Car repair history in last 2 years: battery, alternator, struts, brakes, balls joints, cam sensor, air conditioning had hole in grill - all replaced, Question 3: Dealer was trying to rip me off? That's what transmission guy suggested. Yes I'm from Venus, cars are not my forte, hence wondering veracity of dealer mechanics' report. PS No engine lights every come on.