1992 Isuzu Trooper

I just bought a 1992 Isuzu trooper 3.2 L SOHC manual transmission. It dies when clutching at high speeds, like when I get off the highway. The check engine light also comes on at times, but not necessarily when it dies. The oil pressure gauge fluctuates from 0 to 55. Rarely at 55, occasionally on highway pressure reaches 55. Seems to coincide with times it dies. The heater core has been bypassed but I don't know why, I assume it leaks. The motor runs cold all the time. It seems to have good power and acceleration. RPMs seem good, overall it runs good except for the stalling.