poor acceleration/shaking at certain RPMs

my '81 diesel rabbit is at it again. the acceleration is very dogged until i reach a certain RPM, where it then smooths out. 2nd gear is usually the worst (of course it is the gear that you are usually accelerating through) when i reach cruising speed (30-40mph), i have to hold it the throttle either a little below where i want it or a little above to keep it from shaking/lurching.. i know thats a poor way to describe it, but the only way i know how.. a little history...my injector pump was leaking so i recently had it resealed and a new timing belt put on. fresh out of the shop, it didnt have this problem. in fact if accelerated unbelievably well...after a couple weeks however, it changed, and this problem presented. also, my fuel tank leaks if i have more than 1/4 tank fuel. havent' had time to patch/swap tanks yet but will soon..thanks for any ideas!