1999 MERCUY GRAND MARQIE while driving at low speeds 25 to 45 mph AC blows cold, when speeds get up to 65 to 75 the AC is blowing warmer air. When speed is back down to low range, AC is blowing cold again.
Start at the basics, get a set of professional gauges and put them on the vehicle to check out what the low and high side pressures are.
The compressor may not be cycling correctly causing this A/C issue. The pressure readings on the gauges will help to confirm this by showing low pressures.
It sounds like you may be low on freon and may need to add a few ounces of R134a freon.
Try these suggestions and get back to me when you can, we can diognosis the problem fruther if needed.