Nissan Sentra B13 Power Problems

Greetings, esteemed forum members. I have a question. My Sentra B13 with an electronic distributor overheated and the head gasket burned out. The gasket was replaced, but now the problem is that it loses power going uphill. The mechanic says it's not the timing because it accelerates right away. We already replaced the fuel pump. However, I do notice that the cable running from the throttle to the transmission seems a bit loose, and when it shifts gears, it does so roughly. Thank you for any help you can provide. Regards.