1993 Saturn SL2 Car Doesn't Seem To Work In The Heat

Recently within the past 2 months, my car has been acting up very strangely. When it tends to get hot outside like now, as that it is summer, my Saturn SL2 starts up fine but has such a high idle in the 2,000 RPM's. When I switch gears to reverse, it hits really hard, I assume to the high RPM's but, when I then put it in drive, the car seems to drive as if something is pulling it back. Driving really slow and still the high idle, the Speedometer does not work nor does the mile count. Though when it's cooler in the day the car starts up and runs perfectly fine with not problems. I've replaced many things on this car such as the Transmission, Alternator, Relays, Fuses, EGR Valve, Many Sensors, Radiator, Water Pump, Thermostat, & even a new computer that I'd gotten flashed from a dealership. I still think possibly they screwed me over with the computer because ever since the computer change, the car runs horrible but, I'm unsure and I'd rather not buy another computer if not need be. If you could help me out by identifying the possible problem I'd really appreciate it. Thank You. ~Corey~