What anti-freeze are you using? Many of the aftermarket anti-freezes on the store shelves are OAT, even a DexCool derivative, they are NOT to be used even if they say They work in every brand/model, mixes with any color anti-freeze. Its a totally bogus claim, and they have lost in court over it, but the court had no power in stopping them, it was just trademark court.
Use the original Green, IAT anti-freeze. If you can't find it, then HOAT should work OK, Zerex G-05 is the Chrysler HOAT. Do NOT use any OAT anti-freezes, and many of these bogus aftermarket anti-freezes are OAT but do NOT say it, they very vaguely allude to being some sort of universal anti-freeze and they are NOT.
If you don't know what has been in the cooling system, flush it thoroughly, make sure all the old coolant is out of it.
Mixing OAT, especially DexCool or its derivative, with IAT (original Green) will cause solids to form. The original Green, if you leave it in the cooling system too long (Its suppose to be changed every 2 years), or you mix it with hard water (tap water from areas with lots of minerals dissolved in the water) it will form solids as well. To many people neglected the original green anti-freeze (which is arguably still the best protecting anti-freeze) and the resulting problems is why we have 2 dozen different long life anti-freeze that have all sorts of different compatibility problems that no one can keep track of.