Preferably an adjustable front shock, but anything tighter than the Calvert's will be better. I use some old Lakewood 90/10s that are a lot stiffer than the Calverts which are REALLY loose. Sounds exactly like what my car did after I put on the Calvert 90/10 front shocks. I got it to hook once on a very well prepped track and "treated" tires...it almost went on the back bumper. The very next pass it started to unload again. Put my old front shocks back on and it went right back to hooking like always. The front rises too fast, bottoms out, and unloads the rear.
I put on new rear shocks, new tires, and every possible bar adjustment trying to fix it...didn't respond to anything. I never thought those Calvert shocks would be the problem b/c everyone says they're great, but I found out the hard way.