A few more thoughts;
My recent Craft Beer & Brewing magazine had a section off flavours and one is a solvent type taste. the main cause is fermenting too hot (at 21c you are at the top end of WLP005) or leaching from plastics (you do say you have a cheap cool box mash tun and I wonder if this could be part of the problem).
I would be inclined to go with a dried yeast such as S04 next time, and maybe give leaf hops a try rather than pellets.
Edit.. Having re-read your original post I notice the first AG was OK. Assuming you are using the same equipment then this may point to the ingredients, especially as you have used the same for the 3 bad brews.
I would therefore be inclined to source a fresh set of ingredients from a different supplier (although I have used The Home Brew Shop for years without any problems) for your next brew. The use of leaf hops will stop any sludge carry over.
My recent Craft Beer & Brewing magazine had a section off flavours and one is a solvent type taste. the main cause is fermenting too hot (at 21c you are at the top end of WLP005) or leaching from plastics (you do say you have a cheap cool box mash tun and I wonder if this could be part of the problem).
I would be inclined to go with a dried yeast such as S04 next time, and maybe give leaf hops a try rather than pellets.
Edit.. Having re-read your original post I notice the first AG was OK. Assuming you are using the same equipment then this may point to the ingredients, especially as you have used the same for the 3 bad brews.
I would therefore be inclined to source a fresh set of ingredients from a different supplier (although I have used The Home Brew Shop for years without any problems) for your next brew. The use of leaf hops will stop any sludge carry over.