I have an '86 volvo 240 dl that is misfiring/"hesitating" around 40mph. The distributor, wires and plugs are only a few months old. The plugs are clean and gapped correctly. It overheated a couple of weeks ago when the thermostat went out and I replaced the waterpump at the same time. A lot of problems disappeared after these repairs were made (intermittent increase in idle, stalling and hard starting) but since then its had the misfire but only (or mostly) at around 40mph in fourth gear. I've also noticed a very slow leak around the rear seal. Ideas? Suggestions? Anyone know what this could be caused by?

(I'm going to do a compression test in a few days)