Redtek is a hydrocarbon refrigerant, and the variable displacement compressor won't work properly with it in the stock configuration. You can have a perfectly healthy charge in there and it won't cool for anything when the car is standing still (but should on the highway). This usually leads to the person doing the charging dumping even more refrigerant in there until it's overcharged. I reckon this is what's happening here. Pressures go too high and then the compressor cuts out. Also, there's a mix of the stuff and R12 in there now... who knows what the system is really doing.
To get the hydrocarbons working properly, the rad fans must come on with the compressor. There should be a pressure transducer on the lines near the condenser with two wires coming out of it - that's the rad fan pressure switch. This switch is expecting R12 pressures, not Redtek, so the fans won't be coming on often enough. What you need to do is wire in a relay so that when the compressor comes on, these wires are shorted - this will kick the rad fans on full time. Then you'll get your cooling when the car isn't moving.
This assumes a properly working system, though. You need to make sure the condensor and evaporator are both free of debris that could be blocking them. Make sure the blend door moves as intended. Find the leak in the AC system and fix it. Your best bet is to probably go find a pro who knows these compressors, have them fix the leak, and then retrofit to 134a. It's likely easier than getting the hydrocarbons working.
If you insist on saving cash and going with the hydrocarbons, forget Redtek and use Duracool. If you're in Canada, it's as close as the nearest Walmart or Princess Auto. It gets closer to the pressures the compressor (but not the rad fan pressure switch) is expecting. Don't do any charging at all until you get the relay mod installed or it won't cool and you'll go right past the stopping point (which on my car was 9-11 degrees Celsius at the center vents with the fan on full speed). Use a full manifold gauge set. I did the job on the Imperial with only a low side gauge on the charging hose after I found myself replacing a leaky low side schrader valve, learning as I went... it was a massive pain in the neck, let me tell you. I documented it on the forums - you should be able to find that thread with a search.
It is also likely that you'll never get the performance you got with R12 on the hydrocarbons. The Imperial doesn't. But that's ok. I always get 9-12 degree vent temps with the fan on full, no matter what. Sometimes I'll see it go down to 7 degrees, but not too often. It can be 15 or 35 degrees outside and it will still do the same vent temps. I've come to accept that as normal operation for the variable displacement system on this refrigerant.
