My first thought was to wonder how anyone could diagnose a faulty vacuum regulator (whatever that is) over the phone, but I guess its only slightly less bizarre than trying to figure this out over the internet. anyway, even though I have absolutely no familiarity with the 420SEL and you're not giving much away but I'd suggest you start with some logical assumptions to try and get on the right track.
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when I start the car at temperture below 80 the fuse for the ac unit blows
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Is this engine temperature or outdoors temp? I'm guessing its engine temp, so what you're saying is when you start the car cold, presumably with the AC on, the fuse blows. What happens then? you change the fuse? will the car start anyway?
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The engines dies on the road most of the time at night or the day when radio or ac is on. I drove it tonight with both off and had no problems.
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Since you're having problems with the AC fuse, try driving with the AC off and radio on and see if that has any effect. I think its unlikely the radio's going to have anything to do with this. You can at least rule it out.
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Since this problem has started the rpms are very low even at 65 mph.
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Now we might be getting somewhere. Are you saying the engine has lost power? Does it go as fast as it used to? How is the acceleration away from a stop? Does the engine pull smoothly like it used to?
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Engine will die at any speed.
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How does it die? Cough and splutter or completely cuts out? the former would suggest a fuel problem, the latter an electrical fault. If it completely cuts out, what happens with the cars electrics? Do the lights go off? Do the dash warning lights come on? Can you start the car immediately or wait for it to cool down?
I think the first thing you have to do is decide if this is an electrical issue - the engine isn't getting spark in one form or other - or mechanical/combustion, in that it isn't getting fuel. However, depending on the fuel system used on your engine, it could be a combination of both.
If its electrical, it could be a bad connection in the ignition circuit. It could be related to the fuel pump relay and AC circuit but you'd need help from someone more familiar with your model of car for that. I'm not sure which components are used on that model.
Decide if the cutting out is related to engine/car temp. If it only happens when the car is hot, this would suggest a connection is breaking down due to heat, either in a resistor or coil. If it happens whether the car is hot or cold, this would suggest an intermittent component failure, like a relay or wiring.
How long have you had this car? Who normally services it? If its a bad cable or relay, you may need to get your hands dirty trying to replicate the problem in the driveway.
Hope this helps.